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>

fs/nilfs2/gcinode.c

@@ -52,8 +52,9 @@
 #include "dat.h"
 #include "ifile.h"
 
-static struct address_space_operations def_gcinode_aops = {};
-/* XXX need def_gcinode_iops/fops? */
+static struct address_space_operations def_gcinode_aops = {
+	.sync_page		= block_sync_page,
+};
 
 /*
  * nilfs_gccache_submit_read_data() - add data buffer and submit read request