ceph: fix overflowed constant issue in ceph_do_objects_copy()

ceph: fix overflowed constant issue in ceph_do_objects_copy()

The Coverity Scan service has detected overflowed constant
issue in ceph_do_objects_copy() [1]. The CID 1624308
defect contains explanation: "The overflowed value due to
arithmetic on constants is too small or unexpectedly
negative, causing incorrect computations. Expression bytes,
which is equal to -95, where ret is known to be equal to -95,
underflows the type that receives it, an unsigned integer
64 bits wide. In ceph_do_objects_copy: Integer overflow occurs
in arithmetic on constant operands (CWE-190)".

The patch changes the type of bytes variable from size_t
to ssize_t with the goal of to be capable to receive
negative values.

[1] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1624308

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>