Revert "xio: make xio_session_write/read_header void"
forgot to commit changes in xio_session.c

This reverts commit 5f353fe8589ea7d1aef227b1e3fe523ea353d076.
xio: make xio_session_write/read_header void

As this 2 functions always return 0, so better change them to void.

Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Eyal Salomon <esalomon@mellanox.com>