xfs: use kmem functions for struct xfs_mount

The remount function uses the kmem functions for allocating and freeing
struct xfs_mount, for consistency use the kmem functions everwhere for
struct xfs_mount.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>