io_uring: only publish fully handled mem region

io_uring: only publish fully handled mem region

io_register_mem_region() can try to remove a region right after
publishing it. This non-atomicity is annoying. Do it in two steps
similar to io_register_mem_region(), create memory first and publish it
once the rest of the handling is done. Remove now unused
io_create_region_mmap_safe(), which was assumed to be a temporary
solution from day one.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>