xfs: get quota inode from mp & flags rather than dqp

Allow us to get the appropriate quota inode from any
mp & quota flags, not necessarily associated with a
particular dqp.  Needed for when we are searching for
the next active ID with quotas and we want to examine
the quota inode.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>