erasure-code: add Ceph version check to plugins

Add the __erasure_code_version function to all plugins, to return the
Ceph version against which they have been compiled. When a plugin is
loaded, an error is thrown if the version of the plugin does not match
the version of the daemon loading it.

If the symbol does not exist, which will be true of older plugins, set
the version to "an older version" so it never matches.

http://tracker.ceph.com/issues/9167 Fixes: #9167

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