A kernel memory fault during a remote write on a nearly full domain is
now prevented.


Typical stack trace:

panic
trap
_XentMM
cfs_delay_adjust
cfs_rwvp_cache
cfs_cachewrite
cfs_write
vn_write
rwuio
write
syscall
_Xsyscall

where the sync_pg argument passed from cfs_rwvp_cache() to
cfs_delay_adjust() is NULL.

