This patchset cleans up direct access to bvec table.
The 1st patch passes bvec table to bio_init(), so that
direct access to bvec table in bio initialization is avoided.
For other patches, most of them uses bio_add_page()
to replace hardcode style of adding page to bvec,
and others avoids to access bio->bi_vcnt.
The most special one is to use bvec iterator helpers
to implement .get_page/.next_page for dm-io.c
One big motivation is to prepare for supporting multipage
bvec, but this patchset is one good cleanup too even not
for that purpose.