dtrace: Fix a kernel panic in printm()

When using printm(), one should always pass a scratch pointer to it.
This is achieved by calling printm with memref

BEGIN { printm(fixed_len, memref(ptr, var_len)); }

which will return a pointer to the DTrace scratch space of size
sizeof(uintptr_t) * 2. However, one can easily call printm() as follows

BEGIN { printm(10, (void *)NULL); }

and panic the kernel as a result. This commit does two things:

(1) adds a new macro DTRACE_INSCRATCHPTR(mstate, ptr, howmany) which
    checks if a certain pointer is in the DTrace scratch space;
(2) uses DTRACE_INSCRATCHPTR() to implement a check on printm()'s DIFO
    return value in order to avoid the panic and sets CPU_DTRACE_BADADDR
    if the address is not in the scratch space.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D41722