LU-2859 osc: unplug IO queue async to avoid stack overflow

LU-2859 osc: unplug IO queue async to avoid stack overflow

Otherwise, there is a stack overflow problem with the following
stacktrace:

18 cfs_trace_unlock_tcd at ffffffffa034336c [libcfs]
19 libcfs_debug_vmsg2 at ffffffffa0354038 [libcfs]
20 libcfs_debug_msg at ffffffffa03545f1 [libcfs]
21 osc_key_init at ffffffffa0a90e17 [osc]
22 keys_fill at ffffffffa06d8c3f [obdclass]
23 lu_context_init at ffffffffa06dcc6b [obdclass]
24 lu_env_init at ffffffffa06dce3e [obdclass]
25 cl_env_new at ffffffffa06e40bd [obdclass]
26 cl_env_get at ffffffffa06e4acb [obdclass]
27 lov_sub_get at ffffffffa0b3777d [lov]
28 lov_page_subio at ffffffffa0b37c5d [lov]
29 lov_page_own at ffffffffa0b311af [lov]
30 cl_page_own0 at ffffffffa06e8b5b [obdclass]
31 cl_page_own_try at ffffffffa06e8db3 [obdclass]
32 discard_pagevec at ffffffffa0a926a9 [osc]
33 osc_lru_shrink at ffffffffa0a935d9 [osc]
34 osc_lru_del at ffffffffa0a94aa6 [osc]
35 osc_page_delete at ffffffffa0a951b4 [osc]
36 cl_page_delete0 at ffffffffa06e99e5 [obdclass]
37 cl_page_delete at ffffffffa06e9e62 [obdclass]
38 ll_releasepage at ffffffffa0bfe41b [lustre]
39 try_to_release_page at ffffffff81110070
40 shrink_page_list.clone.0 at ffffffff8112a501
41 shrink_inactive_list at ffffffff8112a8cb
42 shrink_zone at ffffffff8112b5df
43 zone_reclaim at ffffffff8112c384
44 get_page_from_freelist at ffffffff81122834
45 __alloc_pages_nodemask at ffffffff81123ab1
46 kmem_getpages at ffffffff8115e2f2
47 cache_grow at ffffffff8115e95f
48 cache_alloc_refill at ffffffff8115ebb2
49 __kmalloc at ffffffff8115f8d9
50 cfs_alloc at ffffffffa0344c40 [libcfs]
51 ptlrpc_request_alloc_internal at ffffffffa085d407 [ptlrpc]
52 ptlrpc_request_alloc_pool at ffffffffa085d66e [ptlrpc]
53 osc_brw_prep_request at ffffffffa0a8451b [osc]
54 osc_build_rpc at ffffffffa0a8a513 [osc]
55 osc_io_unplug0 at ffffffffa0aa642d [osc]
56 osc_io_unplug at ffffffffa0aa7ce1 [osc]
57 osc_enter_cache at ffffffffa0aa8473 [osc]
58 osc_queue_async_io at ffffffffa0aae916 [osc]
59 osc_page_cache_add at ffffffffa0a94fc9 [osc]
60 cl_page_cache_add at ffffffffa06e61d7 [obdclass]
61 lov_page_cache_add at ffffffffa0b31325 [lov]
62 cl_page_cache_add at ffffffffa06e61d7 [obdclass]
63 vvp_io_commit_write at ffffffffa0c1161d [lustre]
64 cl_io_commit_write at ffffffffa06f5b1d [obdclass]
65 ll_commit_write at ffffffffa0be68be [lustre]
66 ll_write_end at ffffffffa0bfe4e0 [lustre]
67 generic_file_buffered_write at ffffffff81111684
68 __generic_file_aio_write at ffffffff81112f70
69 generic_file_aio_write at ffffffff8111320f
70 vvp_io_write_start at ffffffffa0c11f3c [lustre]
71 cl_io_start at ffffffffa06f244a [obdclass]
72 cl_io_loop at ffffffffa06f6d54 [obdclass]
73 ll_file_io_generic at ffffffffa0bbda7b [lustre]
74 ll_file_aio_write at ffffffffa0bbdce2 [lustre]
75 ll_file_write at ffffffffa0bbeaac [lustre]
76 vfs_write at ffffffff81177b98
77 sys_write at ffffffff811785a1

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Id61b481fe2036a4d7adb7140c39e50fe61c264ba
Reviewed-on: http://review.whamcloud.com/5526
Tested-by: Hudson
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>