io_uring/memmap: helper for pinning region pages

In preparation to adding kernel allocated regions extract a new helper
that pins user pages.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a17d7c39c3de4266b66b75b2dcf768150e1fc618.1732886067.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>