Clear the kernel grab of the FPU state on fork.  The pcb_save pointer
is already correctly reset to the FPU user save area, only PCB_KERNFPU
flag might leak from old thread state into the new state.

For creation of the user-mode thread, the change is nop since
corresponding syscall code does not use FPU.  On the other hand,
creation of a kernel thread forks from a thread selected arbitrary
from proc0, which might use FPU.

Reported and tested by:	Chris Torek <torek@torek.net>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week