nfsd: don't hold ls_mutex across a layout recall

We do need to serialize layout stateid morphing operations, but we
currently hold the ls_mutex across a layout recall which is pretty
ugly. It's also unnecessary -- once we've bumped the seqid and
copied it, we don't need to serialize the rest of the CB_LAYOUTRECALL
vs. anything else. Just drop the mutex once the copy is done.

This was causing a "workqueue leaked lock or atomic" warning and an
occasional deadlock.

There's more work to be done here but this fixes the immediate
regression.

Fixes: cc8a5532

 "nfsd: serialize layout stateid morphing operations"
Cc: stable@vger.kernel.org
Reported-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>