Revert "dtrace: make 'ring' and 'fill' policies imply 'noswitch' flag"

This reverts commit e92491d95ff3500e140eafa614e88ca84ffb0d26.

The general idea looked good to me. In particular, it allowed to save
some memory and avoid memory allocation failures when a large buffer
size was requested along with ring and fill policies.

But I didn't take into account that the second, supposedly unused
buffer, was actually used as the scratch buffer. The scratch buffer is
used as a temporary space for DTrace subroutines like copyin, copyinstr,
and alloca.

I think that the change can be fixed by allocating a separate smaller
buffer for the scratch buffer, but that fix would require more work than
I am able to do now. Hence the revert.

Reported by: Domagoj Stolfa
Diagnosed by: Domagoj Stolfa, markj
MFC after: immediately