lib/bloom: Fix bloom hashing on 32-bit architectures

lib/bloom: Fix bloom hashing on 32-bit architectures

There was an implicit cast from guint64 to gsize (which is 32-bit on
armhf, for example) before the modulus arithmetic which safely narrows
the index.

Fix that by using a guint64 intermediate variable and making the cast
explicit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1231
Approved by: cgwalters