xfs: kill mutex_t typedef

People continue to complain about this for weird reasons, but there's
really no point in keeping this typedef for a couple of users anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Felix Blyakher <felixb@sgi.com>