fs/proc/kcore.c: use PAGE_ALIGN instead of ALIGN(PAGE_SIZE)

Use mm.h definition.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>