nilfs2: unfold nilfs_dat_inode function

nilfs_dat_inode function was a wrapper to switch between normal dat
inode and gcdat, a clone of the dat inode for garbage collection.

This function got obsolete when the gcdat inode was removed, and now
we can access the dat inode directly from a nilfs object.  So, we will
unfold the wrapper and remove it.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 95e4443..ad4ac60 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -650,12 +650,11 @@
 
 int nilfs_count_free_blocks(struct the_nilfs *nilfs, sector_t *nblocks)
 {
-	struct inode *dat = nilfs_dat_inode(nilfs);
 	unsigned long ncleansegs;
 
-	down_read(&NILFS_MDT(dat)->mi_sem);	/* XXX */
+	down_read(&NILFS_MDT(nilfs->ns_dat)->mi_sem);
 	ncleansegs = nilfs_sufile_get_ncleansegs(nilfs->ns_sufile);
-	up_read(&NILFS_MDT(dat)->mi_sem);	/* XXX */
+	up_read(&NILFS_MDT(nilfs->ns_dat)->mi_sem);
 	*nblocks = (sector_t)ncleansegs * nilfs->ns_blocks_per_segment;
 	return 0;
 }