checkpatch: reduce is_maintained_obsolete lookup runtime

The is_maintained_obsolete function can be called twice using the same
filename.  This function spawns a process using get_maintainer.pl.
Store the status of each filename when spawned and use the stored result
to eliminate the spawning of unnecessary duplicate child processes.

Example:

old:

  $ time ./scripts/checkpatch.pl hp100-Move-to-staging.patch > /dev/null

  real	0m1.767s
  user	0m1.634s
  sys	0m0.141s

new:

  $ time ./scripts/checkpatch.pl hp100-Move-to-staging.patch > /dev/null

  real	0m1.184s
  user	0m1.085s
  sys	0m0.103s

Link: http://lkml.kernel.org/r/b982566a2b9b4825badce36fdfc3032bd0005151.camel@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>