reactor: implement read_some(fd, buffer, len) in io_uring
reactor: implement read_some(fd, buffer, len) in io_uring

now that io_uring retries the op returns -EAGAIN if the fd being
accessed was opened with O_NONBLOCK. we don't need to wait until
the POLLIN for read from the fd using the io_uring read op.

in this change, `reactor_backend_uring::read_some(pollable_fd_state& fd,
void* buffer, size_t len)` is also implemented with the speculated
read, like the other overload `read_some()` methods in this class.

Signed-off-by: Kefu Chai <cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>

syuu1228/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar