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 c152737 ("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>

Author: idryomov

fs/ceph/super.c

@@ -431,6 +431,8 @@ static int ceph_parse_mount_param(struct fs_context *fc,
 
 	switch (token) {
 	case Opt_snapdirname:
+		if (strlen(param->string) > NAME_MAX)
+			return invalfc(fc, "snapdirname too long");
 		kfree(fsopt->snapdir_name);
 		fsopt->snapdir_name = param->string;
 		param->string = NULL;