mds/cephfs_features: print size_t using "%zu"

mds/cephfs_features: print size_t using "%zu"

to silence the warning from GCC like:

../src/mds/cephfs_features.cc: In function 'void cephfs_dump_features(ceph::Formatter*, const feature_bitset_t&)':
../src/mds/cephfs_features.cc:71:39: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'size_t' {aka 'long long unsigned int'} [-Wformat=]
   71 |     snprintf(s, sizeof(s), "feature_%lu", i);
      |                                     ~~^   ~
      |                                       |   |
      |                                       |   size_t {aka long long unsigned int}
      |                                       long unsigned int
      |                                     %llu

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