f2fs: should cover i_xattr_nid with its xattr node page lock

Previously, f2fs_setxattr assigns i_xattr_nid in the inode page inconsistently.

The scenario is:

= Thread 1 =         = Thread 2 =     = fi->i_xattr_nid =  = on-disk nid =

f2fs_setxattr                                   0                 0
  new_node_page                                 X                 0
                   sync_inode_page              X                 X
                   checkpoint                   X                 X -.
    grab_cache_page                             X                 X  |
--> allocate a new xattr node block or -ENOSPC      <----------------'

At this moment, the checkpoint stores inconsistent data where the inode has
i_xattr_nid but actual xattr node block is not allocated yet.

So, we should assign the real i_xattr_nid only after its xattr node block is
allocated.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 858a333..1c21344 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -851,6 +851,9 @@
 	SetPageUptodate(page);
 	set_page_dirty(page);
 
+	if (ofs == XATTR_NODE_OFFSET)
+		F2FS_I(dn->inode)->i_xattr_nid = dn->nid;
+
 	dn->node_page = page;
 	if (ipage)
 		update_inode(dn->inode, ipage);