afs: Fix creation calls in the dynamic root to fail with EOPNOTSUPP

[ Upstream commit 1da4bd9f9d187f53618890d7b66b9628bbec3c70 ]

Fix the lookup method on the dynamic root directory such that creation
calls, such as mkdir, open(O_CREAT), symlink, etc. fail with EOPNOTSUPP
rather than failing with some odd error (such as EEXIST).

lookup() itself tries to create automount directories when it is invoked.
These are cached locally in RAM and not committed to storage.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
Tested-by: Jonathan Billings <jsbillings@jsbillings.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>