osd: allow sparse reads with a non-zero truncate-seq

osd: allow sparse reads with a non-zero truncate-seq

do_read() just uses the truncate_seq to tell how to cap the length of
the read. I see no reason that sparse reads should do anything
differently.

Change do_sparse_read() to cap the requested length at the truncate_size
if the truncate_seq in the request is newer than the one in the object.

Fixes: https://tracker.ceph.com/issues/54280
Signed-off-by: Jeff Layton <jlayton@redhat.com>