Suppress unused variable warning for zfs_fm.c

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

sys/contrib/openzfs/module/zfs/zfs_fm.c:256:6: error: variable 'cnt' set but not used [-Werror,-Wunused-but-set-variable]
        int cnt = 0;
            ^

The 'cnt' variable does not seem to be used for anything, but since this
is contributed code, suppress the warning instead.

MFC after: 3 days