nilfs2: use i_blocksize()

Since i_blocksize() helper has been defined in fs.h, use it instead of
open-coding.

Link: http://lkml.kernel.org/r/1485184655-3895-3-git-send-email-konishi.ryusuke@lab.ntt.co.jp
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/nilfs2/btree.c b/fs/nilfs2/btree.c
index 9c7ea17..06ffa13 100644
--- a/fs/nilfs2/btree.c
+++ b/fs/nilfs2/btree.c
@@ -119,7 +119,7 @@
 
 static int nilfs_btree_node_size(const struct nilfs_bmap *btree)
 {
-	return 1 << btree->b_inode->i_blkbits;
+	return i_blocksize(btree->b_inode);
 }
 
 static int nilfs_btree_nchildren_per_block(const struct nilfs_bmap *btree)