locks: generic_delete_lease doesn't need a file_lock at all

Ensure that it's OK to pass in a NULL file_lock double pointer on
a F_UNLCK request and convert the vfs_setlease F_UNLCK callers to
do just that.

Finally, turn the BUG_ON in generic_setlease into a WARN_ON_ONCE
with an error return. That's a problem we can handle without
crashing the box if it occurs.

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