common/rc: don't unset previous SKIP_REASON in _have_kernel_option()

common/rc: don't unset previous SKIP_REASON in _have_kernel_option()

unset previous SKIP_REASON could cause a false failure.

nvme/033 => nvme0n1 (create and connect to an NVMeOF target with a passthru controller) [failed]
    runtime  2.383s  ...  0.376s
    --- tests/nvme/033.out      2022-06-08 08:46:27.956692453 +0000
    +++ /home/lizhijian/blktests/results/nvme0n1/nvme/033.out.bad       2022-06-15 06:32:31.131001276 +0000
    @@ -1,3 +1,8 @@
     Running nvme/033
    -NQN:blktests-subsystem-1 disconnected 1 controller(s)
    +tests/nvme/rc: line 181: nvme: command not found
    +tests/nvme/033: line 19: nvme: command not found
    +tests/nvme/033: line 20: nvme: command not found
    +tests/nvme/033: line 19: nvme: command not found
    +tests/nvme/033: line 20: nvme: command not found
    ...
    (Run 'diff -u tests/nvme/033.out /home/lizhijian/blktests/results/nvme0n1/nvme/033.out.bad' to see the entire diff)

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>