sys_util: netlink: use better naming for netlink message length
sys_util: netlink: use better naming for netlink message length

The hdr_len was actually keeping length of the netlink message including
header (nlmsg) which was misleading. Rename it to msg_len which is more
accurate.

BUG=b:197247746
TEST=Build with 'USE=crosvm-direct emerge-${BOARD} crosvm'

Change-Id: Id4adcd038d41960eeeb8bda3623c8b3b129ef9df
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3375543
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm