btrfs: open code set_extent_bits

This helper calls set_extent_bit with two more parameters set to default
values, but otherwise it's purpose is not clear.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>