ext4: set csum seed in tmp inode while migrating to extents

When migrating to extents, the temporary inode will have it's own checksum
seed.  This means that, when swapping the inodes data, the inode checksums
will be incorrect.

This can be fixed by recalculating the extents checksums again.  Or simply
by copying the seed into the temporary inode.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=213357Reported-by: Jeroen van Wolffelaar <jeroen@wolffelaar.nl>
Signed-off-by: Luís Henriques <lhenriques@suse.de>
Link: https://lore.kernel.org/r/20211214175058.19511-1-lhenriques@suse.deSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/migrate.c

@@ -459,6 +459,17 @@ int ext4_ext_migrate(struct inode *inode)
 		ext4_journal_stop(handle);
 		goto out_unlock;
 	}
+	/*
+	 * Use the correct seed for checksum (i.e. the seed from 'inode').  This
+	 * is so that the metadata blocks will have the correct checksum after
+	 * the migration.
+	 *
+	 * Note however that, if a crash occurs during the migration process,
+	 * the recovery process is broken because the tmp_inode checksums will
+	 * be wrong and the orphans cleanup will fail.
+	 */
+	ei = EXT4_I(inode);
+	EXT4_I(tmp_inode)->i_csum_seed = ei->i_csum_seed;
 	i_size_write(tmp_inode, i_size_read(inode));
 	/*
 	 * Set the i_nlink to zero so it will be deleted later
@@ -502,7 +513,6 @@ int ext4_ext_migrate(struct inode *inode)
 		goto out_tmp_inode;
 	}
 
-	ei = EXT4_I(inode);
 	i_data = ei->i_data;
 	memset(&lb, 0, sizeof(lb));