[NET]: restructure sock_aio_{read,write} / sock_{readv,writev}

[NET]: restructure sock_aio_{read,write} / sock_{readv,writev}

Mid-term I plan to restructure the file_operations so that we don't need
to have all these duplicate aio and vectored versions.  This patch is
a small step in that direction but also a worthwile cleanup on it's own:

(1) introduce a alloc_sock_iocb helper that encapsulates allocating a
    proper sock_iocb
(2) add do_sock_read and do_sock_write helpers for common read/write
    code

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>