nilfs2: zero fill unused portion of super root block

The super root block is newly-allocated each time it is written back
to disk, so unused portion of the block should be cleared.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 8006d0c..8a2b428 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -889,12 +889,14 @@
 {
 	struct buffer_head *bh_sr;
 	struct nilfs_super_root *raw_sr;
-	unsigned isz = nilfs->ns_inode_size;
+	unsigned isz, srsz;
 
 	bh_sr = NILFS_LAST_SEGBUF(&sci->sc_segbufs)->sb_super_root;
 	raw_sr = (struct nilfs_super_root *)bh_sr->b_data;
+	isz = nilfs->ns_inode_size;
+	srsz = NILFS_SR_BYTES(isz);
 
-	raw_sr->sr_bytes = cpu_to_le16(NILFS_SR_BYTES(isz));
+	raw_sr->sr_bytes = cpu_to_le16(srsz);
 	raw_sr->sr_nongc_ctime
 		= cpu_to_le64(nilfs_doing_gc() ?
 			      nilfs->ns_nongc_ctime : sci->sc_seg_ctime);
@@ -906,6 +908,7 @@
 				 NILFS_SR_CPFILE_OFFSET(isz), 1);
 	nilfs_write_inode_common(nilfs->ns_sufile, (void *)raw_sr +
 				 NILFS_SR_SUFILE_OFFSET(isz), 1);
+	memset((void *)raw_sr + srsz, 0, nilfs->ns_blocksize - srsz);
 }
 
 static void nilfs_redirty_inodes(struct list_head *head)