fbarray: reduce duplication in contiguous finding

fbarray: reduce duplication in contiguous finding

Mostly a code move, to have all code related to find_contig in
one place. This slightly changes the API in that previously,
calling find_contig_free() on a full fbarray would've been
an error, but equivalent call to find_contig_used() on an empty
array does not return an error, leading to an inconsistency in
the API.

The decision was made to not treat this condition as an error,
because it is equivalent to calling find_contig() on an index
that just happens to be used/free, which is not an error and
will return 0.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>