gfs2: Cancel remote delete work asynchronously

In gfs2_inode_lookup and gfs2_create_inode, we're calling
gfs2_cancel_delete_work which currently cancels any remote delete work
(delete_work_func) synchronously.  This means that if the work is
currently running, it will wait for it to finish.  We're doing this to
pevent a previous instance of an inode from having any influence on the
next instance.

However, delete_work_func uses gfs2_inode_lookup internally, and we can
end up in a deadlock when delete_work_func gets interrupted at the wrong
time.  For example,

  (1) An inode's iopen glock has delete work queued, but the inode
      itself has been evicted from the inode cache.

  (2) The delete work is preempted before reaching gfs2_inode_lookup.

  (3) Another process recreates the inode (gfs2_create_inode).  It tries
      to cancel any outstanding delete work, which blocks waiting for
      the ongoing delete work to finish.

  (4) The delete work calls gfs2_inode_lookup, which blocks waiting for
      gfs2_create_inode to instantiate and unlock the new inode =>
      deadlock.

It turns out that when the delete work notices that its inode has been
re-instantiated, it will do nothing.  This means that it's safe to
cancel the delete work asynchronously.  This prevents the kind of
deadlock described above.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>