lib/scatterlist: use consistent sg_copy_buffer() return type

sg_copy_buffer() returns a size_t with the number of bytes copied.
Return 0 instead of false if the copy is skipped.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>