From: Hugh Dickins <hugh@veritas.com>

bio_copy is used only by the loop driver, which already has to walk the bio
segments itself: so it makes sense to change it from bio.c export to loop.c
static, as prelude to working upon it there.

bio_copy itself is unchanged by this patch, with one exception.  On oom
failure it must use bio_put, instead of mempool_free to static bio_pool:
which it should have been doing all along - it was leaking the veclist.

(Grudgingly acked by Jens)