vfs mount: change char * to const char*

There is no reason that the char* that are passed to mount will not be
const char*.

This is important when using c++ code which is not as forgiving when
using quoted text as string.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>