xfstests: quota not supported on realtime filesystems

xfstests: quota not supported on realtime filesystems

Realtime XFS filesystems do not support quotas, so quota tests
always fail on such filesystems. Add a check to _require_quota to
detect this situation and notrun the quota tests...

Also, fix _require_xfs_quota and _require_prjquota to have the same
checks.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>