hexdump: don't print bytes with bit 7 set

As Herbert Xu pointed out, bytes (chars) with bit 7 (0x80) set are true
with isprint() but they may not be isascii() but be Unicode instead, so
don't try to print them in hex dumps.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>