bitset: Fix BIT_TEST_SET_ATOMIC and BIT_TEST_CLR_ATOMIC.

[mmel]

Summary

The last argument to atomic_testandset/clear_long should be the bit position in the long, not full bitset index.
This completes a fix for 942951ba46ecd5ebab18de006a24dc52e2d3f745 started in D27886.

[rlibby]

Actually atomic(9) says that it will do the mod:

atomic_testandset(p, v)
        bit = 1 << (v % (sizeof(*p) * NBBY));
        tmp = (*p & bit) != 0;
        *p |= bit;
        return (tmp);

[mmel]

Hmm, right. It would be better to read the manual before coding. My bad.

[rlibby]

You're right that 32-bit arm also has a bug though and needs more follow up.