scripts/documentation-file-ref-check: exclude false-positives

scripts/documentation-file-ref-check: exclude false-positives

There are at least two cases where a documentation file was gone
for good, but the text still mentions it:

1) drivers/vhost/vhost.c:
   the reference for Documentation/virtual/lguest/lguest.c is just
   to give credits to the original work that vhost replaced;

2) Documentation/scsi/scsi_mid_low_api.txt:
   It gives credit and mentions the old Documentation/Configure.help
   file that used to be part of Kernel 2.4.x

As we don't want to keep the script to keep pinpoint to those
every time, let's add a logic at the script to allow it to ignore
valid false-positives like the above.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>