try_get_cap_refs can be used as a condition in a wait_event* calls.
This is all fine until it has to call __ceph_do_pending_vmtruncate,
which in turn acquires the i_truncate_mutex. This leads to a situation
in which a task's state is !TASK_RUNNING and at the same time it's
trying to acquire a sleeping primitive. In essence a nested sleeping
primitives are being used. This causes the following warning:

WARNING: CPU: 22 PID: 11064 at kernel/sched/core.c:7631 __might_sleep+0x9f/0xb0()
do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffff8109447d>] prepare_to_wait_event+0x5d/0x110
 ipmi_msghandler tcp_scalable ib_qib dca ib_mad ib_core ib_addr ipv6
CPU: 22 PID: 11064 Comm: fs_checker.pl Tainted: G           O    4.4.20-clouder2 #6
Hardware name: Supermicro X10DRi/X10DRi, BIOS 1.1a 10/16/2015
 0000000000000000 ffff8838b416fa88 ffffffff812f4409 ffff8838b416fad0
 ffffffff81a034f2 ffff8838b416fac0 ffffffff81052b46 ffffffff81a0432c
 0000000000000061 0000000000000000 0000000000000000 ffff88167bda54a0
Call Trace:
 [<ffffffff812f4409>] dump_stack+0x67/0x9e
 [<ffffffff81052b46>] warn_slowpath_common+0x86/0xc0
 [<ffffffff81052bcc>] warn_slowpath_fmt+0x4c/0x50
 [<ffffffff8109447d>] ? prepare_to_wait_event+0x5d/0x110
 [<ffffffff8109447d>] ? prepare_to_wait_event+0x5d/0x110
 [<ffffffff8107767f>] __might_sleep+0x9f/0xb0
 [<ffffffff81612d30>] mutex_lock+0x20/0x40
 [<ffffffffa04eea14>] __ceph_do_pending_vmtruncate+0x44/0x1a0 [ceph]
 [<ffffffffa04fa692>] try_get_cap_refs+0xa2/0x320 [ceph]
 [<ffffffffa04fd6f5>] ceph_get_caps+0x255/0x2b0 [ceph]
 [<ffffffff81094370>] ? wait_woken+0xb0/0xb0
 [<ffffffffa04f2c11>] ceph_write_iter+0x2b1/0xde0 [ceph]
 [<ffffffff81613f22>] ? schedule_timeout+0x202/0x260
 [<ffffffff8117f01a>] ? kmem_cache_free+0x1ea/0x200
 [<ffffffff811b46ce>] ? iput+0x9e/0x230
 [<ffffffff81077632>] ? __might_sleep+0x52/0xb0
 [<ffffffff81156147>] ? __might_fault+0x37/0x40
 [<ffffffff8119e123>] ? cp_new_stat+0x153/0x170
 [<ffffffff81198cfa>] __vfs_write+0xaa/0xe0
 [<ffffffff81199369>] vfs_write+0xa9/0x190
 [<ffffffff811b6d01>] ? set_close_on_exec+0x31/0x70
 [<ffffffff8119a056>] SyS_write+0x46/0xa0

This happens since wait_event_interruptible can interfere with the
mutex locking code, since they both fiddle with the task state.

Fix the issue by using the newly-added nested blocking infrastructure
in 61ada528 ("sched/wait: Provide infrastructure to deal with
nested blocking")

Link: https://lwn.net/Articles/628628/Signed-off-by: NNikolay Borisov <kernel@kyup.com>
Signed-off-by: NYan, Zheng <zyan@redhat.com>