orangefs: make wait_for_direct_io() take iov_iter

incidentally, insane or compromised server returning *more* than
requested on read should not oops the kernel - initialize the
iov_iter for read according to the iovec we've got.  That's why
pvfs_bufmap_copy_to_iovec() needed a separate size argument - we
shouldn't abuse iov_iter_count(iter) for passing that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>