bitmap: use unsigned int for iterating over bitmap words

bitmap: use unsigned int for iterating over bitmap words

Bitmap words (e.g. resulting from BITMAP_{N,HEAD}WORDS) are of
unsigned type.

Use "unsigned int" to iterate over bitmap words to avoid comparisons
between signed and unsigned expressions. GCC otherwise warns about
these when -Wsign-compare is enabled.

Signed-off-by: Emilio G. Cota <cota@braap.org>