ext4: remove dynamic array size in ext4_chksum()

The ext4_checksum() inline function was using a dynamic array size,
which is not legal C.  (It is a gcc extension).

Remove it.

Cc: "Darrick J. Wong" <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 1610e80..e8e8afa 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1667,10 +1667,12 @@
 {
 	struct {
 		struct shash_desc shash;
-		char ctx[crypto_shash_descsize(sbi->s_chksum_driver)];
+		char ctx[4];
 	} desc;
 	int err;
 
+	BUG_ON(crypto_shash_descsize(sbi->s_chksum_driver)!=sizeof(desc.ctx));
+
 	desc.shash.tfm = sbi->s_chksum_driver;
 	desc.shash.flags = 0;
 	*(u32 *)desc.ctx = crc;