get_maintainer: repair STDIN usage

get_maintainer: repair STDIN usage

Commit 22dd5b0cba50a197aaa3bd2790a29ee2e8e4e372 (fix perlcritic warnings)
broke the ability to handle STDIN because the three argument version of
open() cannot handle standard IO-streams (which is mentioned in
PerlBestPractices, too).

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>