LU-10227 ptlrpc: simplify struct ptlrpc_request_set

LU-10227 ptlrpc: simplify struct ptlrpc_request_set

Remove obd_statfs_rqset(), replacing its only use with
obd_statfs(). Collapse lov_statfs_async() and lov_statfs() into a
single function, removing the need for lov_statfs_interpret().

Remove the then unused set_wakeup_ptr, set_cblist, set_interpret, and
set_arg members of struct ptlrpc_request_set. Remove struct
ptlrpc_set_cbdata and ptlrpc_set_add_cb(). On x86_64 this reduces the
size of struct ptlrpc_request_set from 152 bytes to 112.

Add sanity test_118n() to ensure that OST_STATFS requests are still
async.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Iec8aa378157367f03de96f82d67158b281ec374c
Reviewed-on: https://review.whamcloud.com/30060
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>