[netdevice] Ensure driver transmit() and poll() will not be re-entered

[netdevice] Ensure driver transmit() and poll() will not be re-entered

When CONSOLE_SYSLOG is used, a DBG() from within a network device
driver may cause its transmit() or poll() methods to be unexpectedly
re-entered.  Since these methods are not intended to be re-entrant,
this can lead to undefined behaviour.

Add an explicit re-entrancy guard to both methods.  Note that this
must operate at a per-netdevice level, since there are legitimate
circumstances under which the netdev_tx() or netdev_poll() functions
may be re-entered (e.g. when using VLAN devices).

Signed-off-by: Michael Brown <mcb30@ipxe.org>