ceph: WQ_NON_REENTRANT is meaningless and going away

dbf2576e37 ("workqueue: make all workqueues non-reentrant") made
WQ_NON_REENTRANT no-op and the flag is going away.  Remove its usages.

This patch doesn't introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Sage Weil <sage@inktank.com>
Cc: ceph-devel@vger.kernel.org
ceph: fix freeing inode vs removing session caps race

remove_session_caps() uses iterate_session_caps() to remove caps,
but iterate_session_caps() skips inodes that are being deleted.
So session->s_nr_caps can be non-zero after iterate_session_caps()
return.

We can fix the issue by waiting until deletions are complete.
__wait_on_freeing_inode() is designed for the job, but it is not
exported, so we use lookup inode function to access it.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>