Fix unused variable warning in mmu_radix.c

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

sys/powerpc/aim/mmu_radix.c:5409:22: error: variable 'freed' set but not used [-Werror,-Wunused-but-set-variable]
        int allfree, field, freed, 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

(cherry picked from commit 3446738e8deb6d475f88ca16fbb7b11473d17010)