ext4: fix an endianness bug in ext4_encrypted_zeroout()

ex->ee_block is not host-endian (note that accesses of other fields
of *ex right next to that line go through the helpers that do proper
conversion from little-endian to host-endian; it might make sense
to add similar for ->ee_block to avoid reintroducing that kind of
bugs...)

Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>