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

tests/zfs-tests/include/commands.cfg

@@ -184,6 +184,7 @@ export ZFS_FILES='zdb
     arc_summary
     arcstat
     dbufstat
+    mount.zfs
     zed
     zgenhostid
     zstream