xfs: merge freeing of mp names and mp

In all cases when struct xfs_mount (mp) fields m_rtname and m_logname
are freed mp is also freed, so merge these into a single function
xfs_mount_free()

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>