libceph: add sparse read support to msgr1

libceph: add sparse read support to msgr1

Add 2 new fields to ceph_connection_v1_info to track the necessary info
in sparse reads. Skip initializing the cursor for a sparse read.

Break out read_partial_message_section into a wrapper around a new
read_partial_message_chunk function that doesn't zero out the crc first.

Add new helper functions to drive receiving into the destinations
provided by the sparse_read state machine.

Signed-off-by: Jeff Layton <jlayton@kernel.org>