nfsd: fix nfsd_file_unhash_and_dispose

nfsd: fix nfsd_file_unhash_and_dispose

nfsd_file_unhash_and_dispose() is called for two reasons:

We're either shutting down and purging the filecache, or we've gotten a
notification about a file delete, so we want to go ahead and unhash it
so that it'll get cleaned up when we close.

We're either walking the hashtable or doing a lookup in it and we
don't take a reference in either case. What we want to do in both cases
is to try and unhash the object and put it on the dispose list if that
was successful. If it's no longer hashed, then we don't want to touch
it, with the assumption being that something else is already cleaning
up the sentinel reference.

Instead of trying to selectively decrement the refcount in this
function, just unhash it, and if that was successful, move it to the
dispose list. Then, the disposal routine will just clean that up as
usual.

Also, just make this a void function, drop the WARN_ON_ONCE, and the
comments about deadlocking since the nature of the purported deadlock
is no longer clear.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>