io_uring/net: use REQ_F_IMPORT_BUFFER for send_zc

Instead of a bool field in struct io_sr_msg, use REQ_F_IMPORT_BUFFER to
track whether io_send_zc() has already imported the buffer. This flag
already serves a similar purpose for sendmsg_zc and {read,write}v_fixed.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20250325143943.1226467-1-csander@purestorage.com


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