Use a mutex to synchronize the driver top and bottom halves instead of
using critcal_enter() and critical_exit() to attempt to replace spl*()
calls. The critical section was calling selrecord(), which locks an
MTX_DEF mutex, which is not legal in a critical section.

Tested by: Stefan Ehmann <shoesoft@gmx.net> and "make universe"
Approved by: re (scottl)