nilfs2: get rid of NILFS_I_NILFS

This replaces all references of NILFS_I_NILFS(inode)->ns_bdev with
inode->i_sb->s_bdev and unfolds remaining uses of NILFS_I_NILFS inline
function.

Before 2.6.37, referring to a nilfs object from inodes needed a
conditional judgement, and NILFS_I_NILFS was helpful to simplify it.
But now we can simply do it by going through a super block instance
like inode->i_sb->s_fs_info.

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

fs/nilfs2/sufile.h

@@ -31,7 +31,7 @@
 
 static inline unsigned long nilfs_sufile_get_nsegments(struct inode *sufile)
 {
-	return NILFS_I_NILFS(sufile)->ns_nsegments;
+	return ((struct the_nilfs *)sufile->i_sb->s_fs_info)->ns_nsegments;
 }
 
 unsigned long nilfs_sufile_get_ncleansegs(struct inode *sufile);