libzfs: import: zpool_clear_label: actually fail if clearing l2arc header fails
Found with -Wunused-but-set-variable on Clang trunk
Upstream-commit: a4e0cee1780cbd8f2cb9a263a0ed8d91dbe68b4a
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13304