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

man/man8/zfs.8

@@ -4177,7 +4177,9 @@ allow            subcommand     Must also have the permission that is
                                 being allowed
 clone            subcommand     Must also have the 'create' ability and
                                 'mount' ability in the origin file system
-create           subcommand     Must also have the 'mount' ability
+create           subcommand     Must also have the 'mount' ability.
+                                Must also have the 'refreservation' ability to
+                                create a non-sparse volume.
 destroy          subcommand     Must also have the 'mount' ability
 diff             subcommand     Allows lookup of paths within a dataset
                                 given an object number, and the ability