Mark the return address of the call to ast() in the generic trap

Mark the return address of the call to ast() in the generic trap
handling code so the stack trace unwinders don't start trying to
go into user-space.

Found by trying to create core dumps with a KTR_COMPILE/KTR_GEOM
kernel, which results in a stack_save() call in the ast() coredump
path - this created a panic, and then calling 'trace' in ddb resulted
in the black screen of death after printing out most of the backtrace.