block/008: avoid _offline_cpu() call in sub-shell

block/008: avoid _offline_cpu() call in sub-shell

The helper function _offline_cpu() sets a value to RESTORE_CPUS_ONLINE.
However, the commit bd6b882b2650 ("block/008: check CPU offline failure
due to many IRQs") put _offline_cpu() call in sub-shell, then the set
value to RESTORE_CPUS_ONLINE no longer affects function caller's
environment. This resulted in offlined CPUs not restored by _cleanup()
when the test case block/008 calls only _offline_cpu() and does not call
_online_cpu().

To fix the issue, avoid _offline_cpu() call in sub-shell. Use file
redirect to get output of _offline_cpu() instead of sub-shell execution.

Fixes: bd6b882b2650 ("block/008: check CPU offline failure due to many IRQs")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Link: https://lore.kernel.org/linux-block/20220703180956.2922025-1-yi.zhang@redhat.com/
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>