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/btnode.c b/fs/nilfs2/btnode.c
index 5115814c..388e9e8 100644
--- a/fs/nilfs2/btnode.c
+++ b/fs/nilfs2/btnode.c
@@ -104,8 +104,7 @@
 	if (pblocknr == 0) {
 		pblocknr = blocknr;
 		if (inode->i_ino != NILFS_DAT_INO) {
-			struct inode *dat =
-				nilfs_dat_inode(NILFS_I_NILFS(inode));
+			struct inode *dat = NILFS_I_NILFS(inode)->ns_dat;
 
 			/* blocknr is a virtual block number */
 			err = nilfs_dat_translate(dat, blocknr, &pblocknr);