f2fs: Change the timing of f2fs_wait_on_page_writeback

some backing devices need pages to be stable during writeback. It doesn't matter if
the page is completely overwritten or already uptodate, it needs to wait before write.

---
 fs/f2fs/data.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -1386,13 +1386,13 @@ put_next:
 	f2fs_put_dnode(&dn);
 	f2fs_unlock_op(sbi);
 
+	f2fs_wait_on_page_writeback(page, DATA);
+
 	if (len == PAGE_CACHE_SIZE)
 		goto out_update;
 	if (PageUptodate(page))
 		goto out_clear;
 
-	f2fs_wait_on_page_writeback(page, DATA);
-
 	if ((pos & PAGE_CACHE_MASK) >= i_size_read(inode)) {
 		unsigned start = pos & (PAGE_CACHE_SIZE - 1);
 		unsigned end = start + len;

--
1.7.9.5

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