fscrypt: fix the test_dummy_encryption mount option

fscrypt: fix the test_dummy_encryption mount option

Commit 2375da2442e6b: "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: 2375da2442e6bb472e33406a54fa8f09f091956a
Cc: stable@vger.kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>