madvise: correct the comment of MADV_DODUMP flag

s/MADV_NODUMP/MADV_DONTDUMP/

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>