ubifs: use IS_ENCRYPTED() instead of ubifs_crypt_is_encrypted()

ubifs: use IS_ENCRYPTED() instead of ubifs_crypt_is_encrypted()

There's no need for the ubifs_crypt_is_encrypted() function anymore.
Just use IS_ENCRYPTED() instead, like ext4 and f2fs do.  IS_ENCRYPTED()
checks the VFS-level flag instead of the UBIFS-specific flag, but it
shouldn't change any behavior since the flags are kept in sync.

Link: https://lore.kernel.org/r/20191209212721.244396-1-ebiggers@kernel.org
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Eric Biggers <ebiggers@google.com>