fscrypt: fix the test_dummy_encryption mount option

Commit f1c131b4: "crypto: xts - Convert to skcipher" now fails
the setkey operation if the AES key is the same as the tweak key.
Previously this check was only done if FIPS mode is enabled.  Now this
check is also done if weak key checking was requested.  This is
reasonable, but since we were using the dummy key which was a constant
series of 0x42 bytes, it now caused dummy encrpyption test mode to
fail.

Fix this by using 0x42... and 0x24... for the two keys, so they are
different.

Fixes: f1c131b4
Cc: stable@vger.kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>