io_uring: fix merge error in checking send/recv addr2 flags

With the dropping of the IOPOLL checking in the per-opcode handlers,
we inadvertently left two checks in the recv/recvmsg and send/sendmsg
prep handlers for the same thing, and one of them includes addr2 which
holds the flags for these opcodes.

Fix it up and kill the redundant checks.

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