dtrace/arm64: Fix enumeration of FBT return probes

On arm64, the FBT provider treats tail calls as return probes. Ignoring
the question of whether this is really correct, the implementation is
wrong: instr is a pointer to uint32_t, so the removed multiplication by
the instruction size is wrong. As a result, FBT would create return
probes for intra-function branches.

MFC after: 2 weeks
Sponsored by: Innovate UK