selftests: iou-zcrx: Clean up build warnings for error format

Clean up two build warnings:

[1]

iou-zcrx.c: In function ‘process_recvzc’:
iou-zcrx.c:263:37: warning: too many arguments for format [-Wformat-extra-args]
  263 |                         error(1, 0, "payload mismatch at ", i);
      |                                     ^~~~~~~~~~~~~~~~~~~~~~

[2] Use "%zd" for ssize_t type as better

iou-zcrx.c: In function ‘run_client’:
iou-zcrx.c:357:47: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘ssize_t’ {aka ‘long int’} [-Wformat=]
  357 |                         error(1, 0, "send(): %d", sent);
      |                                              ~^   ~~~~
      |                                               |   |
      |                                               int ssize_t {aka long int}
      |                                              %ld

Signed-off-by: Haiyue Wang <haiyuewa@163.com>
Reviewed-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20250502175136.1122-1-haiyuewa@163.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>