ext4 crypto: use per-inode tfm structure

As suggested by Herbert Xu, we shouldn't allocate a new tfm each time
we read or write a page.  Instead we can use a single tfm hanging off
the inode's crypt_info structure for all of our encryption needs for
that inode, since the tfm can be used by multiple crypto requests in
parallel.

Also use cmpxchg() to avoid races that could result in crypt_info
structure getting doubly allocated or doubly freed.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4_crypto.h

@@ -74,13 +74,11 @@ struct ext4_encryption_key {
 } __attribute__((__packed__));
 
 struct ext4_crypt_info {
-	unsigned char	ci_size;
 	char		ci_data_mode;
 	char		ci_filename_mode;
 	char		ci_flags;
 	struct crypto_ablkcipher *ci_ctfm;
 	struct key	*ci_keyring_key;
-	char		ci_raw[EXT4_MAX_KEY_SIZE];
 	char		ci_master_key[EXT4_KEY_DESCRIPTOR_SIZE];
 };
 
@@ -89,7 +87,6 @@ struct ext4_crypt_info {
 #define EXT4_WRITE_PATH_FL			      0x00000004
 
 struct ext4_crypto_ctx {
-	struct crypto_tfm *tfm;         /* Crypto API context */
 	union {
 		struct {
 			struct page *bounce_page;       /* Ciphertext page */