nilfs2: fix style problems in nilfs2_fs.h

This kills the following checkpatch warnings:

WARNING: please, no space before tabs
+^I__le32^Is_first_ino; ^I^I/* First non-reserved inode */$

WARNING: please, no space before tabs
+^I__le16  s_inode_size; ^I^I/* Size of an inode */$

WARNING: please, no space before tabs
+^Ichar^Is_volume_name[16]; ^I/* volume name */$

WARNING: please, no space before tabs
+^Ichar^Is_last_mounted[64]; ^I/* directory where last mounted */$
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>