netlink: drop (int) cast on length arg in NLMSG_OK

The NLMSG_OK macro compares three things:
 - the len arg from the user
 - a size_t: sizeof(struct nlmsghdr)
 - an int: sizeof(struct nlmsghdr) casted
 - an u32: the nlmsghdr->nlmsg_len member

When building with -Wsign-compare, this macro triggers a signed compare
warning.  This is because it compares len to an int, and then compares
it to a u32.  If len is signed, we get a warning due to the last test.
If len is unsigned, we get a warning due to the first test.  Like in
strace:
socketutils.c:145:8: warning: comparison between signed and unsigned
                              integer expressions [-Wsign-compare]

Lets drop the int cast on the first sizeof.  This way, once the user
casts len to an unsigned value, everything shakes out correctly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>