Disable free space checking in savecore. It's file_size estimate is
so wildly conservative that it's basically useless IMHO. The estimate does not
consider the effects of sparseness (~ 3.5x space reduction), ZFS compression (~
32x space reduction), or gzip compression (~ 68x space reduction).
It is an open question what savecore's behavior should be when it actually runs
out of space. This patch implements no special handling. Do you think that
savecore should leave a fragmentary vmcore file behind or delete it on an
ENOSPC or EDQUOT failure?