bio_vec-backed iov_iter

New variant of iov_iter - ITER_BVEC in iter->type, backed with
bio_vec array instead of iovec one.  Primitives taught to deal
with such beasts, __swap_write() switched to using that kind
of iov_iter.

Note that bio_vec is just a <page, offset, length> triple - there's
nothing block-specific about it.  I've left the definition where it
was, but took it from under ifdef CONFIG_BLOCK.

Next target: ->splice_write()...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>