libceph: init the cursor when preparing sparse read in msgr2

[ Upstream commit 321e3c3d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>