nilfs2: enlarge s_volume_name member in nilfs_super_block

Current s_volume_name has 16 bytes, which is too small as modern filesystem.

s_last_mounted resides just after s_volume_name and has 64 bytes.

s_last_mounted is historically came from ext2, but not used in nilfs2 at all.
Deleting s_last_mounted member and merging that space with s_volume_name
enlarge s_volume_name upto 80 bytes for volume label.

When user land tools see the old header for new disk, it will just ignore
additional bytes stored in s_last_mounted.  While, old disk format has only
16 bytes label, it doesn't affects in case seeing the new header for old disk.

Signed-off-by: Jiro SEKIBA <jir@unicus.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

include/linux/nilfs2_fs.h

@@ -207,8 +207,7 @@ struct nilfs_super_block {
 	__le16	s_segment_usage_size;	/* Size of a segment usage */
 
 	__u8	s_uuid[16];		/* 128-bit uuid for volume */
-	char	s_volume_name[16];	/* volume name */
-	char	s_last_mounted[64];	/* directory where last mounted */
+	char	s_volume_name[80];	/* volume name */
 
 	__le32  s_c_interval;           /* Commit interval of segment */
 	__le32  s_c_block_max;          /* Threshold of data amount for