f2fs: add un/freeze_fs into super_operations

This patch supports ioctl FIFREEZE and FITHAW to snapshot filesystem.
Before calling f2fs_freeze, all writers would be suspended and sync_fs
would be completed. So no f2fs has to do something.
Just background gc operation should be skipped due to generate dirty
nodes and data until unfreeze.

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
index 340a21b..62433c6 100644
--- a/fs/f2fs/inode.c
+++ b/fs/f2fs/inode.c
@@ -278,6 +278,7 @@
 	if (inode->i_nlink || is_bad_inode(inode))
 		goto no_delete;
 
+	sb_start_intwrite(inode->i_sb);
 	set_inode_flag(F2FS_I(inode), FI_NO_ALLOC);
 	i_size_write(inode, 0);
 
@@ -285,6 +286,7 @@
 		f2fs_truncate(inode);
 
 	remove_inode_page(inode);
+	sb_end_intwrite(inode->i_sb);
 no_delete:
 	clear_inode(inode);
 }