selftests: net: csum: Fix printk format in recv_get_packet_csum_status()

selftests: net: csum: Fix printk format in recv_get_packet_csum_status()

Following warning is encountered when building selftests on powerpc/32.

  CC       csum
csum.c: In function 'recv_get_packet_csum_status':
csum.c:710:50: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'size_t' {aka 'unsigned int'} [-Wformat=]
  710 |                         error(1, 0, "cmsg: len=%lu expected=%lu",
      |                                                ~~^
      |                                                  |
      |                                                  long unsigned int
      |                                                %u
  711 |                               cm->cmsg_len, CMSG_LEN(sizeof(struct tpacket_auxdata)));
      |                               ~~~~~~~~~~~~
      |                                 |
      |                                 size_t {aka unsigned int}
csum.c:710:63: warning: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'unsigned int' [-Wformat=]
  710 |                         error(1, 0, "cmsg: len=%lu expected=%lu",
      |                                                             ~~^
      |                                                               |
      |                                                               long unsigned int
      |                                                             %u

cm->cmsg_len has type __kernel_size_t and CMSG() macro has the type
returned by sizeof() which is size_t.

size_t is 'unsigned int' on some platforms and 'unsigned long' on
other ones so use %zu instead of %lu.

The code in question was introduced by
commit 91a7de85600d ("selftests/net: add csum offload test").

Signed-off-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/8b69b40826553c1dd500d9d25e45883744f3f348.1768556791.git.chleroy@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>