f2fs: allow write page cache when writting cp

This patch allow write data to normal file when writting
new checkpoint.

We relax three limitations for write_begin path:
1. data allocation
2. node allocation
3. variables in checkpoint

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 17126a2..49434f9 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1918,6 +1918,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	mutex_init(&sbi->gc_mutex);
 	mutex_init(&sbi->cp_mutex);
 	init_rwsem(&sbi->node_write);
+	init_rwsem(&sbi->node_change);
 
 	/* disallow all the data/node/meta page writes */
 	set_sbi_flag(sbi, SBI_POR_DOING);