block: put the same page when adding it to bio

block: put the same page when adding it to bio

When the added page is merged to last same page in bio_add_pc_page(),
the user may need to put this page for avoiding page leak.

bio_map_user_iov() needs this kind of handling, and now it deals with
it by itself in hack style.

Moves the handling of put page into __bio_add_pc_page(), so
bio_map_user_iov() may be simplified a bit, and maybe more users
can benefit from this change.

Cc: Omar Sandoval <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>