[XFS] refactor xfs_mount_free

xfs_mount_free mostly frees the perag data, which is something that is
duplicated in the mount error path.

Move the XFS_QM_DONE call to the caller and remove the useless
mutex_destroy/spinlock_destroy calls so that we can re-use it for the
mount error path. Also rename it to xfs_free_perag to reflect what it
does.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31836a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>