libceph: bump CEPH_PG_MAX_SIZE to 32

... to accommodate potentially very wide EC pools.  This increases the
size of a typical rbd ceph_osd_request by ~12% (from 1040 to 1168 bytes),
but I'd rather go future proof here.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>