include/linux/kdev_t.h: old/new_valid_dev() can return bool

Make old/new_valid_dev return bool due to these two particular functions
only using either one or zero as their return value.

No functional change.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>