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