generic/590: skip test if kernel doesn't support realtime

generic/590: skip test if kernel doesn't support realtime

IMHO, if kernel doesn't supprt realtime, we should skip test.
So add it. Also, when we use _scratch_mkfs on xfs, we will get
the following error:
mkfs failed with extra mkfs options added to "-bsize=4096" by test 590 **

This failure occurs because we have used "export XFS_MKFS_OPTIONS=
${XFS_MKFS_OPTIONS:=-bsize=4096}" in common/config, we don't need to
set it again in extra_options, so remove it.

Signed-off-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>