if_start routines cannot currently be entered without Giant. When
the kernel is running with debug.mpsafenet != 0, this will defer
if_start execution to a task queue thread holding Giant, which may
introduce additional latency, but avoid incorrect execution.
Suggested by: dfr
in all USB ethernet drivers. The qdat structure contains a pointer
to the interface's struct ifnet and is used to process incoming
packets, so simultaneous use of two similar devices caused crashes
and confusion.
The if_udav driver appeared in the tree since Daan's PR, so I made
similar changes to that driver too.
PR: kern/59290
Submitted by: Daan Vreeken <Danovitsch@Vitsch.net>