l2_packet: Add build option to disable Linux packet socket workaround

l2_packet: Add build option to disable Linux packet socket workaround

Linux packet socket workaround(*) has an impact in performance when the
workaround socket needs to be kept open to receive EAPOL frames. While
this is normally avoided with a kernel that has the issue addressed by
closing the workaround packet socket when detecting a frame through the
main socket, it is possible for that mechanism to not be sufficient,
e.g., when an open network connection (no EAPOL frames) is used.

Add a build option (CONFIG_NO_LINUX_PACKET_SOCKET_WAR=y) to disable the
workaround. This build option is disabled by default and can be enabled
explicitly on distributions which have an older kernel or a fix for the
kernel regression.

Also remove the unused variable num_rx.

(*) Linux kernel commit 576eb62598f10c8c7fd75703fe89010cdcfff596
('bridge: respect RFC2863 operational state') from 2012 introduced a
regression for using wpa_supplicant with EAPOL frames and a station
interface in a bridge.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>