Add ShellCheck's --enable=all inside scripts/

Strengthen static code analysis for shell scripts.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12914

scripts/commitcheck.sh

@@ -51,7 +51,7 @@ new_change_commit()
         error=1
     fi
 
-    return $error
+    return "$error"
 }
 
 is_coverity_fix()
@@ -99,7 +99,7 @@ coverity_fix_commit()
         error=1
     fi
 
-    return $error
+    return "$error"
 }
 
 if [ -n "$1" ]; then