LU-9278 quota: avoid crash when access failed quota proc file

LU-9278 quota: avoid crash when access failed quota proc file

If quota dose not have accounting objects because of failing to
load or unsupported type, @lqp_obj will be NULL, we could not access
directly, add proper handling to avoid following crash

[<ffffffffa0c60730>] ? lprocfs_quota_seq_show+0x460/0x460 [lquota]
[<ffffffff8126bd8f>] proc_reg_open+0x6f/0x100
[<ffffffff811fbfb7>] do_dentry_open+0x1a7/0x2e0
[<ffffffff812a8bfc>] ? security_inode_permission+0x1c/0x30
[<ffffffff8126bd20>] ? proc_reg_release+0x70/0x70
[<ffffffff811fc18f>] vfs_open+0x5f/0xe0
[<ffffffff81209b58>] ? may_open+0x68/0x110
[<ffffffff8120ccfd>] do_last+0x1ed/0x12a0
[<ffffffff8120de72>] path_openat+0xc2/0x490
[<ffffffff8118075b>] ? unlock_page+0x2b/0x30
[<ffffffff8120ffeb>] do_filp_open+0x4b/0xb0
[<ffffffff8121cc77>] ? __alloc_fd+0xa7/0x130
[<ffffffff811fd4f3>] do_sys_open+0xf3/0x1f0
[<ffffffff811fd60e>] SyS_open+0x1e/0x20
[<ffffffff81696d49>] system_call_fastpath+0x16/0x1b

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I8dfc2b35c1f38416ed0f933296bdaa455e117926
Reviewed-on: https://review.whamcloud.com/26277
Tested-by: Jenkins
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>