include/ceph_fs: do not use anonymous aggregate with member having ctor

include/ceph_fs: do not use anonymous aggregate with member having ctor

GCC-10 fails to compile ceph_fs.h, with following error:

In file included from ../src/include/types.h:27,
                 from ../src/msg/msg_types.h:23,
                 from ../src/common/ipaddr.cc:16:
../src/include/ceph_fs.h:891:11: error: member ‘ceph_le32 ceph_mds_caps_body_legacy::<unnamed union>::<unnamed struct>::truncate_seq’ with constructor not allowed in anonymous aggregate
  891 |    __le32 truncate_seq;
      |           ^~~~~~~~~~~~
../src/include/ceph_fs.h:892:25: error: member ‘ceph_timespec ceph_mds_caps_body_legacy::<unnamed union>::<unnamed struct>::mtime’ with constructor not allowed in anonymous aggregate
  892 |    struct ceph_timespec mtime, atime, ctime;
      |                         ^~~~~
../src/include/ceph_fs.h:892:32: error: member ‘ceph_timespec ceph_mds_caps_body_legacy::<unnamed union>::<unnamed struct>::atime’ with constructor not allowed in anonymous aggregate
  892 |    struct ceph_timespec mtime, atime, ctime;
      |                                ^~~~~
../src/include/ceph_fs.h:892:39: error: member ‘ceph_timespec ceph_mds_caps_body_legacy::<unnamed union>::<unnamed struct>::ctime’ with constructor not allowed in anonymous aggregate
  892 |    struct ceph_timespec mtime, atime, ctime;
      |                                       ^~~~~

because, for instance, `ceph_timespec` has compiler generated
constructor, and it's not allowed by C++17 standard.

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