hv_sock: Report EOF instead of -EIO for FIN

hv_sock: Report EOF instead of -EIO for FIN

Commit f0c5827d07cb unluckily causes a regression for the FIN packet,
and the final read syscall gets an error rather than 0.

Ideally, we would want to fix hvs_channel_readable_payload() so that it
could return 0 in the FIN scenario, but it's not good for the hv_sock
driver to use the VMBus ringbuffer's cached priv_read_index, which is
internal data in the VMBus driver.

Fix the regression in hv_sock by returning 0 rather than -EIO.

Fixes: f0c5827d07cb ("hv_sock: Return the readable bytes in hvs_stream_has_data()")
Cc: stable@vger.kernel.org
Reported-by: Ben Hillis <Ben.Hillis@microsoft.com>
Reported-by: Mitchell Levy <levymitchell0@gmail.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260416191433.840637-1-decui@microsoft.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>