ext4 crypto: shrink size of the ext4_crypto_ctx structure

Some fields are only used when the crypto_ctx is being used on the
read path, some are only used on the write path, and some are only
used when the structure is on free list.  Optimize memory use by using
a union.

Change-Id: I66de766a0f1122463edf3280ff0c2923be2472b8
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>