checkpatch: improve octal permissions tests

Add world writable permissions tests for the various functions like
debugfs etc...

Add $String type for $FuncArg so that string constants are matched.

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>