iov_iter: Add a segmented queue of bio_vec[]

Add a segmented queue of bio_vec[].  This allows an indefinite quantity of
elements to be handled.  A rolling buffer can also be implemented over them
where new bio_vecs can be added to the leading edge and spent bio_vecs can
be cleared from the trailing edge.

Signed-off-by: David Howells <dhowells@redhat.com>