checkpatch: use the correct indentation for which()

I copied the which subroutine from get_maintainer.pl.

Unfortunately, get_maintainer uses a 4 space indentation so use the
proper tab indentation instead.
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>