nilfs2: add sync_page method to page caches of meta data

This applies block_sync_page() function to the sync_page method of
page caches for meta data files, gc page caches, and btree node
buffers.  This is a companion patch of ("nilfs2: enable sync_page
mothod") which applied the function for data pages.

This allows lock_page() for those meta data to unplug pending bio
requests.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/mdt.c b/fs/nilfs2/mdt.c
index 7b7032e..3d3ddb3 100644
--- a/fs/nilfs2/mdt.c
+++ b/fs/nilfs2/mdt.c
@@ -430,6 +430,7 @@
 
 static struct address_space_operations def_mdt_aops = {
 	.writepage		= nilfs_mdt_write_page,
+	.sync_page		= block_sync_page,
 };
 
 static struct inode_operations def_mdt_iops;