Fix unused variable warning in amd64's pmap.c

With clang 15, the following -Werror warning is produced:

sys/amd64/amd64/pmap.c:8274:22: error: variable 'freed' set but not used [-Werror,-Wunused-but-set-variable]
        int allfree, field, freed, i, idx;
                            ^

The 'freed' variable is only used when PV_STATS is defined. Ensure it is
only declared and set in that case.

MFC after: 3 days