checkpatch: see if modified files are marked obsolete in MAINTAINERS

Use get_maintainer to check the status of individual files.  If
"obsolete", suggest leaving the files alone.

Link: http://lkml.kernel.org/r/7ceaa510dc9d2df05ec4b456baed7bb1415550b3.1471889575.git.joe@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Cc: SF Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>