fix mkdir in symlinked folders; closes #151

remove an overly careful safety-check which would refuse creating
directories if the location was outside of the volume's base-path

it is safe to trust `rem` due to `vpath = undot(vpath)` and
a similar check being performed inside `vfs.get` as well,
so this served no purpose

copyparty/httpcli.py

@@ -2999,9 +2999,6 @@ class HttpCli(object):
         vfs, rem = self.asrv.vfs.get(vpath, self.uname, False, True)
         rem = sanitize_vpath(rem, "/")
         fn = vfs.canonical(rem)
-        if not fn.startswith(vfs.realpath):
-            self.log("invalid mkdir %r %r" % (self.gctx, vpath), 1)
-            raise Pebkac(422)
 
         if not nullwrite:
             fdir = os.path.dirname(fn)