nvmeof-mp/rc: Avoid skipping tests due to the expected SKIP_REASON

nvmeof-mp/rc: Avoid skipping tests due to the expected SKIP_REASON

In _have_kernel_option(), SKIP_REASON = "kernel option NVME_MULTIPATH
has not been enabled" is expected but all nvmeof-mp tests are skipped
due to the SKIP_REASON. For example:
-----------------------------------------------------
./check nvmeof-mp/001
nvmeof-mp/***                                                [not run]
    kernel option NVME_MULTIPATH has not been enabled
-----------------------------------------------------

Avoid the issue by unsetting the SKIP_REASON.

Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Fixes: 7ae143852f6c ("common/rc: don't unset previous SKIP_REASON in _have_kernel_option()")
[Shin'ichiro: reduced in-line comment]
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>