af_unix: Defer sock_put() to clean up path in unix_dgram_sendmsg().

When other has SOCK_DEAD in unix_dgram_sendmsg(), we call sock_put() for
it first and then set NULL to other before jumping to the error path.

This is to skip sock_put() in the error path.

Let's not set NULL to other and defer the sock_put() to the error path
to clean up the labels later.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>