af_unix: Return errno instead of NULL in unix_create1().

unix_create1() returns NULL on error, and the callers assume that it never
fails for reasons other than out of memory.  So, the callers always return
-ENOMEM when unix_create1() fails.

However, it also returns NULL when the number of af_unix sockets exceeds
twice the limit controlled by sysctl: fs.file-max.  In this case, the
callers should return -ENFILE like alloc_empty_file().

This patch changes unix_create1() to return the correct error value instead
of NULL on error.

Out of curiosity, the assumption has been wrong since 1999 due to this
change introduced in 2.2.4 [0].

  diff -u --recursive --new-file v2.2.3/linux/net/unix/af_unix.c linux/net/unix/af_unix.c
  --- v2.2.3/linux/net/unix/af_unix.c	Tue Jan 19 11:32:53 1999
  +++ linux/net/unix/af_unix.c	Sun Mar 21 07:22:00 1999
  @@ -388,6 +413,9 @@
   {
   	struct sock *sk;

  +	if (atomic_read(&unix_nr_socks) >= 2*max_files)
  +		return NULL;
  +
   	MOD_INC_USE_COUNT;
   	sk = sk_alloc(PF_UNIX, GFP_KERNEL, 1);
   	if (!sk) {

[0]: https://cdn.kernel.org/pub/linux/kernel/v2.2/patch-2.2.4.gz

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>