EC: Fix bug of OIV processing XAttr.

When we need to use OIV to print EC information for a directory, `PBImageTextWriter#getErasureCodingPolicyName` will be called. Currently, this method uses `XATTR_ERASURECODING_POLICY.contains(xattr.getName())` to filter and obtain EC XAttr, which is very dangerous. If we have an XAttr whose name happens to be a substring of `hdfs.erasurecoding.policy`, then `getErasureCodingPolicyName` will return the wrong result. Our internal production environment has customized some XAttrs, and this bug caused errors in the parsing results of OIV when using `-ec` option.