af_unix: Add test for SCM_INQ on partial read

Add test to verify that when a skb is partially consumed,
unix_inq_len() return correct remaining byte count.

Before:

  #  RUN           scm_inq.stream.partial_read ...
  # scm_inq.c:165:partial_read:Expected remain (512) == *(int *)CMSG_DATA(cmsg) (768)
  # partial_read: Test terminated by assertion
  #          FAIL  scm_inq.stream.partial_read
  not ok 2 scm_inq.stream.partial_read

After:

  #  RUN           scm_inq.stream.partial_read ...
  #            OK  scm_inq.stream.partial_read
  ok 2 scm_inq.stream.partial_read

Signed-off-by: Jianyu Li <jianyu.li@mediatek.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260601113640.231897-3-jianyu.li@mediatek.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>