common/rc: check error case and fail the test

common/rc: check error case and fail the test

If we place /var/lib/xfstests on a read-only filesystem, commands in
_link_out_file_named() fail to modify the files. However, they won't fail
the test. As a result, the test case fails mysteriously with only "no
qualified output" printed.

Fix it by checking the error case.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>