ceph: messenger: change read_partial() to take "end" arg

(cherry picked from commit fd51653f78cf40a0516e521b6de22f329c5bad8d)

Make the second argument to read_partial() be the ending input byte
position rather than the beginning offset it now represents.  This
amounts to moving the addition "to + size" into the caller.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>