io_uring/rsrc: remove null check on import

WARN_ON_ONCE() checking imu for NULL in io_import_fixed() is in the hot
path and too protective, it's time to get rid of it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3782c01e311dbd474bca45aefaf79a2f2822fafb.1745083025.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.c

@@ -1058,8 +1058,6 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 	size_t offset;
 	int ret;
 
-	if (WARN_ON_ONCE(!imu))
-		return -EFAULT;
 	ret = validate_fixed_range(buf_addr, len, imu);
 	if (unlikely(ret))
 		return ret;