msg/msg_types.h: do not cast `ceph_entity_name` to `entity_name_t` for printing

msg/msg_types.h: do not cast `ceph_entity_name` to `entity_name_t` for printing

in GCC-9, `-Waddress-of-packed-member` is enabled, so we have warnings like:

src/msg/msg_types.h:142:41: warning: converting a packed 'const
ceph_entity_name' pointer (alignment 1) to a 'const entity_name_t'
pointer (alignment 8) may result in an unaligned pointer value
[-Waddress-of-packed-member]
  142 |   return out << *(const entity_name_t*)&addr;
      |                                         ^~~~

since the alignment of these two structures are different, we cannot
cast a structure with the alignment of 1 to a structure with the
alignment of 8. as the code generated by compiler accessing the members
of alignment 8 won't work with the members of alignment 1, we need to
create a temporary structure for printing it.

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