common/rc: handle the case when syscall isn't available

common/rc: handle the case when syscall isn't available

_require_xfs_io_command() isn't handling the case where the
requested syscall isn't available. To fix this simply check the
error returned by xfs_io.

Usually the userspace should be synced with kernelspace, the case
that a syscall is supported by userspace tool but not kernelspace
should not happen, but in rare test setups it's possible, e.g.
building an initramfs that contains the testing tools from a recent
distro, and running the tests against an old kernel (which does not
include such syscall).

[ eguan: it's not copy_file_range syscall specific issue, update
summary and commit log, and provide more background information ]

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>