checkpatch: update octal permissions warning

When checking permissions, make sure 4 octal digits are used, but allow
a single 0 too.

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>