fscrypt: add ->ci_inode to fscrypt_info

Add an inode back-pointer to 'struct fscrypt_info', such that
inode->i_crypt_info->ci_inode == inode.

This will be useful for:

1. Evicting the inodes when a fscrypt key is removed, since we'll track
   the inodes using a given key by linking their fscrypt_infos together,
   rather than the inodes directly.  This avoids bloating 'struct inode'
   with a new list_head.

2. Simplifying the per-file key setup, since the inode pointer won't
   have to be passed around everywhere just in case something goes wrong
   and it's needed for fscrypt_warn().
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/fscrypt_private.h

@@ -73,6 +73,9 @@ struct fscrypt_info {
 	 */
 	struct fscrypt_mode *ci_mode;
 
+	/* Back-pointer to the inode */
+	struct inode *ci_inode;
+
 	/*
 	 * If non-NULL, then this inode uses a master key directly rather than a
 	 * derived key, and ci_ctfm will equal ci_master_key->mk_ctfm.

fs/crypto/keyinfo.c

@@ -556,6 +556,8 @@ int fscrypt_get_encryption_info(struct inode *inode)
 	if (!crypt_info)
 		return -ENOMEM;
 
+	crypt_info->ci_inode = inode;
+
 	crypt_info->ci_flags = ctx.flags;
 	crypt_info->ci_data_mode = ctx.contents_encryption_mode;
 	crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;