mm/util: make memdup_user_nul() similar to memdup_user()

Since the string data to copy from userspace is likely less than PAGE_SIZE
bytes, replace GFP_KERNEL with GFP_USER like commit 6c2c97a2
("memdup_user(): switch to GFP_USER") does and add __GFP_NOWARN like
commit 6c8fcc09 ("mm: don't let userspace spam allocations warnings")
does.  Also, use dedicated slab buckets like commit d73778e4
("mm/util: Use dedicated slab buckets for memdup_user()") does.

Link: https://lkml.kernel.org/r/014cd694-cc27-4a07-a34a-2ae95d744515@I-love.SAKURA.ne.jp


Reported-by:  <syzbot+7e12e97b36154c54414b@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=7e12e97b36154c54414b


Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>