mm: make do_madvise() available internally

This is in preparation for enabling this functionality through io_uring.
Add a helper that is just exporting what sys_madvise() does, and have the
system call use it.

No functional changes in this patch.

Change-Id: I77394d51a06aeb36e0ba733787dbd71a8e06a582
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Git-commit: db08ca25253d56f1f76eb4b3fe32a7ac1fbab741
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>