common: unintended use of the wrong bloom_filter prototype

For encoding/decoding test purposes. The generate_test_instances should
be improved to cover more encoding/decoding cases.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #697 from dachary/master

This lets "make check" pass, so I like it.
Reviewed-by: Greg Farnum <greg@inktank.com>