Fix refreserv_00[24]

tests/sys/cddl/zfs/tests/refreserv/refreserv_002_pos.ksh
Import a fix from Illumos. Quotas and reservations are a little bit
approximate. This change incrementally raises refreservation as
high as it can go, instead of assuming that it can equal the quota.

tests/sys/cddl/zfs/tests/refreserv/refreserv_004_pos.ksh
Instead of assuming that refreservation can equal the parent's
quota, allow a margin of 5MB, like refreserv_001_pos does. I don't
know why this wasn't a problem for Illumos. Also, delete the final
line setting reservation. I'm not sure what the purpose of that
line was. It may originally have been a copy/paste error.

tests/sys/cddl/zfs/tests/refreserv/refreserv_test.sh
Clear expected failures

Obtained from: Illumos (partially)
Sponsored by: Spectra Logic Corp