system-linux: fix race condition in netlink socket error handing

system-linux: fix race condition in netlink socket error handing

The error handling needed for the buffer growth logic relies on
uloop_fd's error flag, which is set based on epoll events. Doing so
without handling recvmsg's error codes is racy, as an error state may be
set between receiving epoll events and the next recvmsg, but calling
recvmsg clears the error state.

To fix this, add handling for errors returned by nl_recvmsgs_default()
and nl_recv(); checking for u->error and retrieving the error status
using getsockopt() becomes redundant.

We have observed this issue on Gluon (recent OpenWrt 23.05); on some
devices with DSA switches, the bridge interface's carrier-on event would
consistenly get lost during boot due to insufficient buffer space
(see [1]).

We have bisected the issue to netifd commit 516ab774cc16 ("system-linux:
fix race condition on bringing up wireless devices"), but that commit only
uncovered the preexisting bug by switching from getting the carrier state
from sysfs to using the netlink messages in cb_rtnl_event().

I suspect that other recent issues about netifd missing a carrier state
change like [2] may have the same underlying cause.

[1] https://github.com/freifunk-gluon/gluon/issues/3130
[2] https://github.com/openwrt/openwrt/issues/13863

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>