checkpatch: speed up checking for filenames in sections marked obsolete

Adding -f to the get_maintainer.pl invocation means git isn't invoked
by get_maintainer.pl for known filenames.

This reduces the overall time to run checkpatch.

Link: http://lkml.kernel.org/r/22991e3a295aeb399b43af0478b6e5809106ccee.1472684066.git.joe@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>