net: enhance decoding of getsockopt's optlen argument

net: enhance decoding of getsockopt's optlen argument

As the last argument of getsockopt syscall has read-write semantics,
print both user and kernel values when they differ.

* net.c (SYS_FUNC(getsockopt)): On entering syscall, fetch and save
the length specified to the kernel.  On error, print the length saved
on entering.  When the saved length and the length returned by the
kernel differ, print both values.
* NEWS: Mention this.
* tests/net-icmp_filter.c (main): Update expected output.