quota: Clean up the namespace in dqblk_xfs.h

Almost all identifiers use the FS_* namespace, so rename the missing few
XFS_* ones to FS_* as well.  Without this some people might get upset
about having too many XFS names in generic code.

Acked-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>