osd/PrimaryLogPG.cc: reassign size only when object size > truncate_size

osd/PrimaryLogPG.cc: reassign size only when object size > truncate_size

Before setting size equal to op.extent.truncate_size, we need to check
if the size of the object is greater than the truncate_size. We do not
need to set size to op.extent.truncate_size, in the case where the size of
the object is less than op.extent.truncate_size.

Without this change, we were always setting size =
op.extent.truncate_size, when (seq < op.extent.truncate_seq) and
(op.extent.offset + op.extent.length > op.extent.truncate_size), were both
true. This ended up in:

1. overestimating the size of the object
2. not considering the correct size of the object, for
   the later checks, which calculate op.extent.length for the read ops
3. causing crashes when trying to read more data than what was present

Fixes: http://tracker.ceph.com/issues/21931
Fixes: http://tracker.ceph.com/issues/22330
Signed-off-by: Neha Ojha <nojha@redhat.com>