msg/Message: use an unsigned type for get_data_len()

msg/Message: use an unsigned type for get_data_len()

to avoid warnings like:

../src/osd/PrimaryLogPG.cc:2123:27: warning: comparison of integer expressions of different signedness: ‘off_t’ {aka ‘long int’} and ‘uint64_t’ {aka ‘long unsigned int’} [-Wsign-compare]
 2123 |         m->get_data_len() > cct->_conf->osd_max_write_size << 20) {
      |         ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

after change the size options' type to unsigned or uint64_t.

Signed-off-by: Kefu Chai <kchai@redhat.com>