Fix memleak in cmd/mount_zfs.c

Convert dynamic allocation to static buffer, simplify parse_dataset
function return path. Add tests specific to the mount helper.

Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Sterling Jensen <sterlingjensen@users.noreply.github.com>
Closes #11098