[LIBS] Fix bitmap_zero(), bitmap_fill(), and bitmap_copy()

We cannot use BIT_WORD() to determine number of longs. Instead, we should
be using BITS_TO_LONGS() to determine number of longs from number of bits.

This patch fixes above described issue in bitmap_zero(), bitmap_fill(),
and bitmap_copy() APIs.

Signed-off-by: Anup Patel <anup@brainfault.org>