This patch fixes the "rm_audit_sync_block: sync block corruption" panic due to
not checking loopback early in boot. We spin through the write_sync loop before
the data is written and panic in the audit code. Then, the data subsequently 
gets written before the dump is written.

Panic String:
    rm_audit_sync_block: sync block corruption

Stack trace:

       0 boot                 src/kernel/arch/alpha/machdep.c : 2964
       1 panic                src/kernel/bsd/subr_prf.c : 1432
       2 rm_crash_nodes       src/kernel/rm/rm_error.c : 2467
       3 rm_panic             src/kernel/rm/rm_error.c : 1915
       4 rm_audit_sync_block  src/kernel/rm/rm_error.c : 1457
       5 rm_write_sync        src/kernel/rm/rm_error.c : 1820
       6 rm_push_shared_data  src/kernel/rm/rm_error.c : 862
       7 rm_eh_init_shared_data_req  src/kernel/rm/rm_prail.c : 928
       8 rm_prail_int         src/kernel/rm/rm_prail.c : 1152
       9 rm_int               src/kernel/rm/rm_prail.c : 1117
      10 Mchan_isr            src/kernel/io/dec/pci/mchan.c : 8077
      11 _XentInt             src/kernel/arch/alpha/locore.s : 1654
      12 idle_thread          src/kernel/kern/sched_prim.c : 4535
