Fix SC2181 ("[ $?") outside tests/

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12042

contrib/dracut/90zfs/zfs-lib.sh.in

@@ -5,12 +5,8 @@ command -v getargbool >/dev/null || {
     # Compatibility with older Dracut versions.
     # With apologies to the Dracut developers.
     getargbool() {
-        if ! [ -z "$_b" ]; then
-                unset _b
-        fi
         _default="$1"; shift
-        _b=$(getarg "$@")
-        [ $? -ne 0 ] &&  [ -z "$_b" ] && _b="$_default"
+        ! _b=$(getarg "$@") && [ -z "$_b" ] && _b="$_default"
         if [ -n "$_b" ]; then
             [ "$_b" = "0" ] && return 1
             [ "$_b" = "no" ] && return 1