ubifs: remove redundant checks for encryption key

In several places, ubifs checked for an encryption key before creating a
file in an encrypted directory.  This was redundant with
fscrypt_setup_filename() or ubifs_new_inode(), and in the case of
ubifs_link() it broke linking to special files.  So remove the extra
checks.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>