librgw: try-implement rgw_readv/rgw_writev

librgw: try-implement rgw_readv/rgw_writev

The model in the rgw_readv call is the caller owns the struct rgw_uio,
and uses it to pass uio_offset.

The caller returns the rgw_uio filled out, and provides indirectly
rgw_uio->uio_vio (the full private data is at uio->uio_p1 on return,
the caller (p==private) must not touch it.  The caller returns
private data in rgw_readv_release() when finished.

Meanwhile rgw_writev is atomic.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>