This patch addresses panics that may be seen due to UBC issues

   crash> tf
   >  0 boot                 src/kernel/arch/alpha/machdep.c : 2717
      1 panic                src/kernel/bsd/subr_prf.c : 1616
      2 simple_lock_fault    src/kernel/kern/lock.c : 2874
      3 mcs_unlock_found_violation  src/kernel/kern/lock.c : 3264
      4 ubc_free_page        src/kernel/vfs/vfs_ubc.c : 8930
      5 ubc_bigpage_alloc    src/kernel/vfs/vfs_ubc.c : 5813
      6 ubc_page_alloc       src/kernel/vfs/vfs_ubc.c : 6178
      7 ubc_lookup           src/kernel/vfs/vfs_ubc.c : 5234
      8 cfs_getapage         src/kernel/tnc_common/tnc_cfe/alpha/cfs_vm_alpha.c :
   1072
      9 cfs_getpages         src/kernel/tnc_common/tnc_cfe/alpha/cfs_vm_alpha.c :   800
     10 cfs_getpage          src/kernel/tnc_common/tnc_cfe/cfs_vm_osi.c : 2207
     11 u_seg_fault          src/kernel/vm/u_mape_seg.c : 1960
     12 u_map_fault          src/kernel/vm/vm_umap.c : 1203
     13 vm_fault             src/kernel/vm/vm_fault.c : 202
     14 trap                 src/kernel/arch/alpha/trap.c : 2352
     15 _XentMM              src/kernel/arch/alpha/locore.s : 2242


   >  0 boot                 src/kernel/arch/alpha/machdep.c : 2717
      1 panic                src/kernel/bsd/subr_prf.c : 1616
      2 trap                 src/kernel/arch/alpha/trap.c : 2289
      3 _XentMM              src/kernel/arch/alpha/locore.s : 2242
      4 ubc_memory_purge     src/kernel/vfs/vfs_ubc.c : 3709
      5 ubc_purge_thread     src/kernel/vfs/vfs_ubc.c : 3977



   >  0 stop_secondary_cpu   src/kernel/arch/alpha/cpu.c : 1416
      1 panic                src/kernel/bsd/subr_prf.c : 1509
      2 event_timeout        src/kernel/arch/alpha/cpu.c : 2430
      3 printf               src/kernel/bsd/subr_prf.c : 1018
      4 panic                src/kernel/bsd/subr_prf.c : 1566
      5 trap                 src/kernel/arch/alpha/trap.c : 2289
      6 _XentMM              src/kernel/arch/alpha/locore.s : 2242
      7 ubc_memory_purge     src/kernel/vfs/vfs_ubc.c : 3722
      8 ubc_purge_thread     src/kernel/vfs/vfs_ubc.c : 3990

