filesystems: add set_nlink()

Replace remaining direct i_nlink updates with a new set_nlink()
updater function.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 07b3ac6..07ea918 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1705,7 +1705,7 @@
 	sb->s_bdi = &fs_info->bdi;
 
 	fs_info->btree_inode->i_ino = BTRFS_BTREE_INODE_OBJECTID;
-	fs_info->btree_inode->i_nlink = 1;
+	set_nlink(fs_info->btree_inode, 1);
 	/*
 	 * we set the i_size on the btree inode to the max possible int.
 	 * the real end of the address space is determined by all of