Move check for prefix path to within cifs_get_root()

commit 348c1bfa84dfc47da1f1234b7f2bf09fa798edea upstream.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Tested-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Acked-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Willy Tarreau <w@1wt.eu>