checkpatch: make utf-8 test --strict

Some patches are sent in using ISO-8859 or even Windows codepage 1252.

Make checkpatch accept these by default and only emit the "Invalid UTF-8"
message when using --strict.

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>