xfs: implement online get/set fs label

The GET ioctl is trivial, just return the current label.

The SET ioctl is more involved:
It transactionally modifies the superblock to write a new filesystem
label to the primary super.

A new variant of xfs_sync_sb then writes the superblock buffer
immediately to disk so that the change is visible from userspace.

It then invalidates any page cache that userspace might have previously
read on the block device so that i.e. blkid can see the change
immediately, and updates all secondary superblocks as userspace relable
does.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[darrick: use dchinner's new xfs_update_secondary_sbs function]
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>