MAINTAINERS: add peterx as userfaultfd reviewer

MAINTAINERS: add peterx as userfaultfd reviewer

Add an entry for userfaultfd and make myself a reviewer of it, just in
case it helps people manage the cc list.

I named it MEMORY USERFAULTFD, could be a bad name, but then it can be
together with the MEMORY* entries when everything is in alphabetic order,
which is definitely a benefit.

The line may not change much on how I'd work with userfaultfd; I think
I'll do the same as before..  But maybe it still, more or less, adds some
responsibility on top, indeed.

[akpm@linux-foundation.org: add include/linux/userfaultfd_k.h, per Mike]
[akpm@linux-foundation.org: fix misordering]
Link: https://lkml.kernel.org/r/20250322002124.131736-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>