fscrypt: mention init_on_free instead of page poisoning

Page poisoning is an older debug option.  The modern way to initialize
memory on free for security reasons is to set init_on_free=1.

Link: https://lore.kernel.org/r/20250304210156.14912-1-ebiggers@kernel.org


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