Btrfs: use the new helper wbc_to_write_flags

This updates btrfs to use the helper wbc_to_write_flags which has been
applied in ext4/xfs/f2fs/block.

Please note that, with this, btrfs's dirty pages written by a
writeback job will carry the flag REQ_BACKGROUND, which is currently
used by writeback-throttle to determine whether it should go to get a
request or wait.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3471,8 +3471,7 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc,
 	unsigned int write_flags = 0;
 	unsigned long nr_written = 0;
 
-	if (wbc->sync_mode == WB_SYNC_ALL)
-		write_flags = REQ_SYNC;
+	write_flags = wbc_to_write_flags(wbc);
 
 	trace___extent_writepage(page, inode, wbc);
 
@@ -3718,7 +3717,7 @@ static noinline_for_stack int write_one_eb(struct extent_buffer *eb,
 	unsigned long i, num_pages;
 	unsigned long bio_flags = 0;
 	unsigned long start, end;
-	unsigned int write_flags = (epd->sync_io ? REQ_SYNC : 0) | REQ_META;
+	unsigned int write_flags = wbc_to_write_flags(wbc) | REQ_META;
 	int ret = 0;
 
 	clear_bit(EXTENT_BUFFER_WRITE_ERR, &eb->bflags);