btrfs: set bbio->file_offset in alloc_new_bio

Instead of digging into the bio_vec in submit_one_bio, set file_offset at
bio allocation time from the provided parameter.  This also ensures that
the file_offset is available all the time when building up the bio
payload.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>