io_uring/rsrc: cleanup io_pin_pages()

This function is overly convoluted with a goto error path, and checks
under the mmap_read_lock() that don't need to be at all. Rearrange it
a bit so the checks and errors fall out naturally, rather than needing
to jump around for it.

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