This patch prevents a variety of panics that may occur during 
low memory conditions when invol_flush_error is set. 

The panics seen so far include: 
    1. Kernel Memory Fault in simple_lock via cfs_rwvp_cache
    2. Unaligned Access in msfs_putpage
    3. Assert Failed: brp->br_allocated>=0 
    4. PANIC: pg_hold - underflow
 


KMF
---
    4 panic 
    5 trap 
    6 _XentMM 
    7 simple_lock 
    8 cfs_rwvp_cache 
    9 cfs_cachewrite 
    10 cfs_write 
    11 vn_write 
    12 rwuio 
    13 write 
    14 syscall 
    15 _Xsyscall 

UNA
---
    4 panic  
    5 afault_trap 
    6 _XentUna    
    7 msfs_putpage 
    8 ubc_msync  
    9 cfs_ubc_msync    
   10 ubc_msync           
   11 u_map_control        
   12 msync           
   13 syscall  
   14 _Xsyscall 

ASSERT FAILED
-------------
    4 panic  
    5 cfsdb_assert 
    6 cfscall_writepages  
    7 cfs_do_bio 
    8 cfs_strategy 
    9 cfs_rwblk               
    10 cfs_rwvp_cache
   11 cfs_cachewrite 
   12 cfs_write 
   13 vn_write
   14 rwuio 
   15 write 
   16 syscall
   17 _Xsyscall

PG_HOLD UNDERFLOW
-----------------
    1 panic
    2 ubc_page_release
    3 cfs_rwvp_cache
    4 cfs_cachewrite
    5 cfs_write
    6 vn_pwrite 
    7 prwuio 
    8 pwrite 
    9 syscall
   10 _Xsyscall



