opr: Allow non-2^x for n_buckets in opr_cache_init

opr: Allow non-2^x for n_buckets in opr_cache_init

Currently, opr_cache_init requires that opts->n_buckets is a power of
2 (since our underlying opr_dict requires this). However, callers may
want to pick a number of buckets based on some other value. Requiring
each caller to calculate the nearest power-of-2 is annoying, so
instead just have opr_cache_init itself calculate a nearby power of 2.

That is, with this commit, opts->n_buckets is allowed to not be a
power of 2; when it's not a power of 2, opr_cache_init will calculate
the next highest power of 2 and use that as the number of buckets.

Change-Id: Icd3c56c1fe0733e3dac964ea9a98ff7b436254e6
Reviewed-on: https://gerrit.openafs.org/14122
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>