checkpatch: improve EMBEDDED_FILENAME test

checkpatch: improve EMBEDDED_FILENAME test

Privately, Heinz Mauelshagen showed that the embedded filename test is not
specific enough.

> WARNING: It's generally not useful to have the filename in the file
> #113: FILE: errors.c:113:
> +            block < registered_errors.blocks + registered_errors.count;

Extend the test to use the appropriate word boundary tests.

Link: https://lkml.kernel.org/r/36069dac5d07509dab1c7f1238f8cbb08db80ac6.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>