InitFatalReboot: Trigger panic explicitly for init_fatal_panic

The exit of init panics the system *after* process context (mm, stack,
...etc.) are recycled, according to Linux kernel's 'do_exit'
implementation. To preserve most init process context for debugging,
triggers the panic via proc-sysrq explicitly.

Note: after this change, there will be no "Attempt to kill init" panic
when androidboot.init_fatal_panic is set.

Test: Insert data abort fault in init, the full process context is
      preserved in memory dump captured after panic.
Bug: 155940351
Change-Id: I3393bd00f99b8cb432cfa19a105b7d636b411764