Add epoch_call_drain() function to wait until scheduled epoch_call() requests will be finished.

[ae]

Summary

I faced with two problems while use epoch(4) KPI with kernel modules:

1. The module creates new epoch with epoch_alloc(). It uses epoch_call() to make deferred free(). When module is unloaded, all scheduled free() do leak.
2. The module uses existing epoch and uses epoch_call() to make deferred free(). When module is unloaded, there is highly likely chance to get memory leak and then page fault, when epoch_cb_task will call method that should make free().

[ae]

It seems this is not enough to prevent panics. Simple kernel module to reproduce the panic.

main.c2 KBDownload