gcc 9.2.0 compiles io_req_find_next() as a separate function leaving
the first REQ_F_LINK_HEAD fast check not inlined. Help it by splitting
out the check from the function.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>