nfsd: always hold the fi_lock when bumping fi_access refcounts

Once we remove the client_mutex, there's an unlikely but possible race
that could occur. It will be possible for nfs4_file_put_access to race
with nfs4_file_get_access. The refcount will go to zero (briefly) and
then bumped back to one. If that happens we set ourselves up for a
use-after-free and the potential for a lock to race onto the i_flock
list as a filp is being torn down.

Ensure that we can safely bump the refcount on the file by holding the
fi_lock whenever that's done. The only place it currently isn't is in
get_lock_access.

In order to ensure atomicity with finding the file, use the
find_*_file_locked variants and then call get_lock_access to get new
access references on the nfs4_file under the same lock.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>