dtrace: correct the aframes value for fbt provider

Set the number of artificial frames to 5:

1. cpu_exception_handler_supervisor()
2. do_trap_supervisor()
3. dtrace_invop_start()
4. dtrace_invop()
5. fbt_invop()

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37663

(cherry picked from commit 36a9ce874b5988dc8615a4b1b3fb41eb9e06051d)