erasure-code: ceph_erasure_code does not need to avoid dlclose

The only reason for not dlclosing plugins at exit is for callgrind but
ceph_erasure_code has no workload that would require callgrind.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
erasure_code: implement ceph_erasure_code to assert the existence of a plugin

This is handy when scripting in the context of teuthology and only
conditionally run tests for the isa plugin, for instance.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>