objecter: set linger op target pg when a linger is resent

objecter: set linger op target pg when a linger is resent

send_linger always creates a new Op, but op_submit does not fill in
the target pg if an existing session is passed in, so when a linger
was resent, it had the wrong pg set.

This caused a crash in cosd with debugging turned on when running
testlibrbd twice. This occurred because the object context for the
linger in the wrong pg had no object name set.