mm/vmalloc.c: is_vmalloc_addr() can return bool

Make is_vmalloc_addr() return bool to improve readability due to this
particular function only using either one or zero as its return value.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>