btrfs: factor check and flush helpers from __btrfsic_submit_bio

Split out two helpers to make __btrfsic_submit_bio more readable.

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