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.

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

fs/ext4/page-io.c

@@ -84,7 +84,7 @@ static void ext4_finish_bio(struct bio *bio)
 			/* The bounce data pages are unmapped. */
 			data_page = page;
 			ctx = (struct ext4_crypto_ctx *)page_private(data_page);
-			page = ctx->control_page;
+			page = ctx->w.control_page;
 		}
 #endif