net: ip: Prevent unaligned exception

s_addr is actually an unsigned integer and it's not guaranteed to be
aligned on 4-byte boundary. In net_ipv4_addr_cmp(), accessing s_addr
directly might cause an unaligned exception on some platform
like xtensa. Use UNALIGNED_GET() to prevent unalgined exception.

Signed-off-by: Aska Wu <aska.wu@linaro.org>