db_nextframe/amd64: catch up with r328083 to recognize fast_syscall_common

db_nextframe/amd64: catch up with r328083 to recognize fast_syscall_common

Since that change the system call stack traces look like this:
  ...
  sys___sysctl() at sys___sysctl+0x5f/frame 0xfffffe0028e13ac0
  amd64_syscall() at amd64_syscall+0x79b/frame 0xfffffe0028e13bf0
  fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe0028e13bf0
So, db_nextframe() stopped recognizing the system call frame.
This commit should fix that.

Reviewed by:	kib
MFC after:	4 days