erasure-code: set max_size to chunk_count() instead of 20

erasure-code: set max_size to chunk_count() instead of 20

The ruleset created for an erasure coded pool has max_size set to a
fixed value of 20, which may be incorrect when more than 20 chunks are
needed and lead to obscure errors. Set it to the number of chunks,
i.e. k+m most of the time.

In a cluster with few OSDs (9 for instance), setting max_size to 20
causes performance problems when injecting a new crushmap. The monitor
will call CrushTester::test which tries 1024 mappins for all sizes
ranging from min_size to max_size. Each attempt to map more OSDs than
available will exhaust all retries (50 by default) and it takes a
significant amount of time. In a cluster with 9 OSDs, testing one such
ruleset can take up to 5 seconds.

Since the test blocks the monitor leader, a few erasure coded rulesets
will block the monitor long enough to exceed the timeouts and trigger an
election.

http://tracker.ceph.com/issues/10363 Fixes: #10363

Signed-off-by: Loic Dachary <ldachary@redhat.com>