[XFS] replace b_fspriv with b_mount

Replace the b_fspriv pointer and it's ugly accessors with a properly types
xfs_mount pointer.  Also switch log reocvery over to it instead of using
b_fspriv for the mount pointer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>