nilfs2: fix regression that i-flag is not set on changeless checkpoints

According to the report from Jiro SEKIBA titled "regression in
2.6.37?"  (Message-Id: <8739n8vs1f.wl%jir@sekiba.com>), on 2.6.37 and
later kernels, lscp command no longer displays "i" flag on checkpoints
that snapshot operations or garbage collection created.

This is a regression of nilfs2 checkpointing function, and it's
critical since it broke behavior of a part of nilfs2 applications.
For instance, snapshot manager of TimeBrowse gets to create
meaningless snapshots continuously; snapshot creation triggers another
checkpoint, but applications cannot distinguish whether the new
checkpoint contains meaningful changes or not without the i-flag.

This patch fixes the regression and brings that application behavior
back to normal.
Reported-by: Jiro SEKIBA <jir@unicus.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Tested-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Tested-by: Jiro SEKIBA <jir@unicus.jp>
Cc: stable <stable@kernel.org>  [2.6.37]

fs/nilfs2/segment.c

@@ -430,7 +430,8 @@ static void nilfs_segctor_begin_finfo(struct nilfs_sc_info *sci,
 	nilfs_segctor_map_segsum_entry(
 		sci, &sci->sc_binfo_ptr, sizeof(struct nilfs_finfo));
 
-	if (inode->i_sb && !test_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags))
+	if (NILFS_I(inode)->i_root &&
+	    !test_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags))
 		set_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags);
 	/* skip finfo */
 }