nilfs2: convert nilfs_bmap_lookup to an inline function

The nilfs_bmap_lookup() is now a wrapper function of
nilfs_bmap_lookup_at_level().

This moves the nilfs_bmap_lookup() to a header file converting it to
an inline function and gives an opportunity for optimization.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/mdt.c b/fs/nilfs2/mdt.c
index 9cb8318..156bf60 100644
--- a/fs/nilfs2/mdt.c
+++ b/fs/nilfs2/mdt.c
@@ -136,7 +136,7 @@
 		       int mode, struct buffer_head **out_bh)
 {
 	struct buffer_head *bh;
-	unsigned long blknum = 0;
+	__u64 blknum = 0;
 	int ret = -ENOMEM;
 
 	bh = nilfs_grab_buffer(inode, inode->i_mapping, blkoff, 0);
@@ -166,7 +166,7 @@
 		goto failed_bh;
 	}
 	bh->b_bdev = NILFS_MDT(inode)->mi_nilfs->ns_bdev;
-	bh->b_blocknr = blknum;
+	bh->b_blocknr = (sector_t)blknum;
 	set_buffer_mapped(bh);
 
 	bh->b_end_io = end_buffer_read_sync;