Btrfs: add decompression code for direct I/O.

The following patch has the btrfs code changes/additions needed to support reading compressed data in my directio code.

The code always runs in process context.

The callbacks:

int (*get_next_in)(struct bio_vec *vec, struct btrfs_inflate *icb); int (*get_next_out)(struct bio_vec *vec, struct btrfs_inflate *icb);

track where we currently are in the compressed pages (get_next_in) and uncompressed output user pages (get_next_out). (in) is always full pages, (out) may be partial pages with offsets as the user input to direct I/O is a set of memory vectors/lengths.

The callback:

void (*done_with_out)(struct bio_vec *vec, struct btrfs_inflate *icb);

is currently coded in directio as:

static void btrfs_dio_done_with_out(struct bio_vec *ovec,
struct btrfs_inflate *icb)

{
	flush_dcache_page(ovec->bv_page);
	if (!PageCompound(ovec->bv_page))
		set_page_dirty_lock(ovec->bv_page);
	page_cache_release(ovec->bv_page);
}

Again, this patch has no value without my directio code.

jim