fscrypt: Add missing superblock check in find_or_insert_direct_key()

fscrypt: Add missing superblock check in find_or_insert_direct_key()

The legacy 'fscrypt_direct_keys' table caches master keys that are used
by v1 encryption policies that have FSCRYPT_POLICY_FLAG_DIRECT_KEY.
It's just a global table for all filesystems (since the keys can be
provided by the legacy process-subscribed keyrings mechanism, which
makes it difficult to reuse super_block::s_master_keys).

The entries in it ('struct fscrypt_direct_key') do contain a super_block
pointer, though, for passing to fscrypt_destroy_inline_crypt_key() when
the last inode that references the key is evicted.

However, when finding the fscrypt_direct_key for an inode, we weren't
actually comparing the super_block pointer.  As a result, inodes with
different super_blocks could point to the same fscrypt_direct_key.  That
could extend the lifetime of a fscrypt_direct_key beyond the
super_block it points to, causing a use-after-free later.

Fix this by creating distinct fscrypt_direct_key structs for distinct
super_block structs.

Note that this problem doesn't exist in the v2 policy equivalent
("per-mode keys"), since the data structures there are per super_block.

Fixes: 22e9947a4b2b ("fscrypt: stop holding extra request_queue references")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260717044303.425265-1-ebiggers%40kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260719033120.122120-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>