io_uring: disable ENTER_EXT_ARG_REG for IOPOLL

IOPOLL doesn't use the extended arguments, no need for it to support
IORING_ENTER_EXT_ARG_REG. Let's disable it for IOPOLL, if anything it
leaves more space for future extensions.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a35ecd919dbdc17bd5b7932273e317832c531b45.1731689588.git.asml.silence@gmail.com


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