af_unix: selftest: Fix the size of the parameter to connect()

Adjust size parameter in connect() to match the type of the parameter, to
fix "No such file or directory" error in selftests/net/af_unix/
test_oob_unix.c:127.

The existing code happens to work provided that the autogenerated pathname
is shorter than sizeof (struct sockaddr), which is why it hasn't been
noticed earlier.

Visible from the trace excerpt:

bind(3, {sa_family=AF_UNIX, sun_path="unix_oob_453059"}, 110) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fa6a6577a10) = 453060
[pid <child>] connect(6, {sa_family=AF_UNIX, sun_path="unix_oob_45305"}, 16) = -1 ENOENT (No such file or directory)

BUG: The filename is trimmed to sizeof (struct sockaddr).

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Cc: Florian Westphal <fw@strlen.de>
Reviewed-by: Florian Westphal <fw@strlen.de>
Fixes: 314001f0 ("af_unix: Add OOB support")
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>