zfs allow refreservation needed for zfs create -V

zfs allow refreservation needed for zfs create -V

When creating a non-sparse volume, zfs create sets a refreservation.
Accordingly, one needs the "refreservation" ability in addition to the
"create" ability in order to create a non-sparse volume.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reported-by: github.com/homerlinux
Reported-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8531
Closes #8624