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/bmap.c b/fs/nilfs2/bmap.c
index 4b7aeb3..3ee67c6 100644
--- a/fs/nilfs2/bmap.c
+++ b/fs/nilfs2/bmap.c
@@ -35,7 +35,7 @@
 
 struct inode *nilfs_bmap_get_dat(const struct nilfs_bmap *bmap)
 {
-	return nilfs_dat_inode(NILFS_I_NILFS(bmap->b_inode));
+	return NILFS_I_NILFS(bmap->b_inode)->ns_dat;
 }
 
 static int nilfs_bmap_convert_error(struct nilfs_bmap *bmap,