jbd2: fix block tag checksum verification brokenness

Al Viro complained of a ton of bogosity with regards to the jbd2 block
tag header checksum.  This one checksum is 16 bits, so cut off the
upper 16 bits and treat it as a 16-bit value and don't mess around
with be32* conversions.  Fortunately metadata checksumming is still
"experimental" and not in a shipping e2fsprogs, so there should be few
users affected by this.
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/jbd2/commit.c

@@ -343,20 +343,21 @@ static void jbd2_block_tag_csum_set(journal_t *j, journal_block_tag_t *tag,
 {
 	struct page *page = bh->b_page;
 	__u8 *addr;
-	__u32 csum;
+	__u32 csum32;
 
 	if (!JBD2_HAS_INCOMPAT_FEATURE(j, JBD2_FEATURE_INCOMPAT_CSUM_V2))
 		return;
 
 	sequence = cpu_to_be32(sequence);
 	addr = kmap_atomic(page);
-	csum = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&sequence,
-			  sizeof(sequence));
-	csum = jbd2_chksum(j, csum, addr + offset_in_page(bh->b_data),
-			  bh->b_size);
+	csum32 = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&sequence,
+			     sizeof(sequence));
+	csum32 = jbd2_chksum(j, csum32, addr + offset_in_page(bh->b_data),
+			     bh->b_size);
 	kunmap_atomic(addr);
 
-	tag->t_checksum = cpu_to_be32(csum);
+	/* We only have space to store the lower 16 bits of the crc32c. */
+	tag->t_checksum = cpu_to_be16(csum32);
 }
 /*
  * jbd2_journal_commit_transaction

fs/jbd2/recovery.c

@@ -399,18 +399,17 @@ static int jbd2_commit_block_csum_verify(journal_t *j, void *buf)
 static int jbd2_block_tag_csum_verify(journal_t *j, journal_block_tag_t *tag,
 				      void *buf, __u32 sequence)
 {
-	__u32 provided, calculated;
+	__u32 csum32;
 
 	if (!JBD2_HAS_INCOMPAT_FEATURE(j, JBD2_FEATURE_INCOMPAT_CSUM_V2))
 		return 1;
 
 	sequence = cpu_to_be32(sequence);
-	calculated = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&sequence,
-				 sizeof(sequence));
-	calculated = jbd2_chksum(j, calculated, buf, j->j_blocksize);
-	provided = be32_to_cpu(tag->t_checksum);
+	csum32 = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&sequence,
+			     sizeof(sequence));
+	csum32 = jbd2_chksum(j, csum32, buf, j->j_blocksize);
 
-	return provided == cpu_to_be32(calculated);
+	return tag->t_checksum == cpu_to_be16(csum32);
 }
 
 static int do_one_pass(journal_t *journal,