checksrc: fix possible endless loop when detecting BANNEDFUNC

If the source line had square brackets before the match, the stripping
of the banned function left the original line intact, and repeated the
check on it forever. E.g. with banned function `open` in `lib518.c`:
```c
t518_testfd[0] = open(DEV_NULL, O_RDONLY);
```

Closes #18775

scripts/checksrc.pl

@@ -902,6 +902,8 @@ sub scanfile {
                       "use of $bad is banned");
             my $replace = 'x' x (length($bad) + 1);
             $prefix =~ s/\*/\\*/;
+            $prefix =~ s/\[/\\[/;
+            $prefix =~ s/\]/\\]/;
             $suff =~ s/\(/\\(/;
             $l =~ s/$prefix$bad$suff/$prefix$replace/;
             goto again;