UPSTREAM: fscrypt: make it clearer that key_prefix is deprecated

fscrypt_operations::key_prefix should not be set by any filesystems that
aren't setting it already.  This is already documented, but apparently
it's not sufficiently clear, as both ceph and btrfs have tried to set
it.  Rename the field to legacy_key_prefix and improve the documentation
to hopefully make it clearer.

Link: https://lore.kernel.org/r/20230925055451.59499-2-ebiggers@kernel.org


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

Bug: 299136786
Bug: 302588300
(cherry picked from commit 5970fbad)
Change-Id: I412dfe8f481b351022eaf1daba6f4138b7c3ffa2
Signed-off-by: Eric Biggers <ebiggers@google.com>