io_uring/cmd: warn on reg buf imports by ineligible cmds

For IORING_URING_CMD_FIXED-less commands io_uring doesn't pull buf_index
from the sqe, so imports might succeed if the index coincide, e.g. when
it's 0, but otherwise it's error prone. Warn if someone tries to import
without the flag.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://lore.kernel.org/r/a1c2c88e53c3fe96978f23d50c6bc66c2c79c337.1747991070.git.asml.silence@gmail.com


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