Convert ubifs to use the new functions fscrypt_prepare_new_inode() and
fscrypt_set_context().

Unlike ext4 and f2fs, this doesn't appear to fix any deadlock bug.  But
it does shorten the code slightly and get all filesystems using the same
helper functions, so that fscrypt_inherit_context() can be removed.

It also fixes an incorrect error code where ubifs returned EPERM instead
of the expected ENOKEY.

Link: https://lore.kernel.org/r/20200917041136.178600-6-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>