cifs: Add missing sentinel to smb3_fs_parameters

Add missing sentinel to smb3_fs_parameters. This fixes ARM32 kernel
crashing once CIFS is registered.

 Unable to handle kernel paging request at virtual address 33626d73
...
 (strcmp) from (fs_validate_description)
 (fs_validate_description) from (register_filesystem)
 (register_filesystem) from (init_cifs [cifs])
 (init_cifs [cifs]) from (do_one_initcall)
 (do_one_initcall) from (do_init_module)
 (do_init_module) from (load_module)
 (load_module) from (sys_finit_module)
 (sys_finit_module) from (ret_fast_syscal)

Fixes: e07724d1cf38 ("cifs: switch to new mount api")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>