xfs: add xfs_mount sysfs kobject

Embed a base kobject into xfs_mount. This creates a kobject associated
with each XFS mount and a subdirectory in sysfs with the name of the
filesystem. The subdirectory lifecycle matches that of the mount. Also
add the new xfs_sysfs.[c,h] source files with some XFS sysfs
infrastructure to facilitate attribute creation.

Note that there are currently no attributes exported as part of the
xfs_mount kobject. It exists solely to serve as a per-mount container
for child objects.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_mount.h

@@ -166,6 +166,7 @@ typedef struct xfs_mount {
 						   on the next remount,rw */
 	int64_t			m_low_space[XFS_LOWSP_MAX];
 						/* low free space thresholds */
+	struct xfs_kobj		m_kobj;
 
 	struct workqueue_struct	*m_data_workqueue;
 	struct workqueue_struct	*m_unwritten_workqueue;