ubifs: remove ubifs_dir_open()

ubifs: remove ubifs_dir_open()

Since encrypted directories can be opened and searched without their key
being available, and each readdir and ->lookup() tries to set up the
key, trying to set up the key in ->open() too isn't really useful.

Just remove it so that directories don't need an ->open() method
anymore, and so that we eliminate a use of fscrypt_get_encryption_info()
(which I'd like to stop exporting to filesystems).

Link: https://lore.kernel.org/r/20201203022041.230976-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>