btrfs: introduce a common helper to calculate the size of a bio

btrfs: introduce a common helper to calculate the size of a bio

We have several call sites doing the same work to calculate the size of
a bio:

	struct bio_vec *bvec;
	u32 bio_size = 0;
	int i;

	bio_for_each_bvec_all(bvec, bio, i)
		bio_size += bvec->bv_len;

We can use a common helper instead of open-coding it everywhere.

This also allows us to constify the @bio_size variables used in all the
call sites.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>