iov_iter: get rid of separate bvec and xarray callbacks

iov_iter: get rid of separate bvec and xarray callbacks

After the previous commit we have
	* xarray and bvec callbacks idential in all cases
	* both equivalent to kvec callback wrapped into
kmap_local_page()/kunmap_local() pair.

So we can pass only two (iovec and kvec) callbacks to
iterate_and_advance() and let iterate_{bvec,xarray} wrap
it into kmap_local_page()/kunmap_local_page().

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