socket: Tighten no-error check in bind()

move_addr_to_kernel() returns only negative values on error, or zero on
success. Rewrite the error check to an idiomatic form to avoid confusing
the reader.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>