libceph: init the cursor when preparing sparse read in msgr2

The cursor is no longer initialized in the OSD client, causing the
sparse read state machine to fall into an infinite loop.  The cursor
should be initialized in IN_S_PREPARE_SPARSE_DATA state.

[ idryomov: use msg instead of con->in_msg, changelog ]

Link: https://tracker.ceph.com/issues/64607
Fixes: 8e46a2d0

 ("libceph: just wait for more data to be available on the socket")
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Tested-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>