generic/574: fix sporadic failure with test_dummy_encryption

generic/574: fix sporadic failure with test_dummy_encryption

When the "test_dummy_encryption" mount option is specified, the
fs-verity file corruption test generic/574 is flaky; it fails about 1%
of the time.  This happens because in the three test cases where a
single byte of the test file is corrupted, sometimes the new byte
happens to be the same as the original.  This is specific to
test_dummy_encryption because the encrypted data is nondeterministic
(and appears random), unlike the unencrypted data.

Fix this by corrupting 5 bytes instead of 1, so that the probability of
failure becomes effectively zero.

Signed-off-by: Eric Biggers <ebiggers@google.com>