ceph: validate snapdirname option length when mounting

It becomes a path component, so it shouldn't exceed NAME_MAX
characters.  This was hardened in commit c152737b ("ceph: Use
strscpy() instead of strcpy() in __get_snap_name()"), but no actual
check was put in place.

Cc: stable@vger.kernel.org
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>