rgw: check for non-existent bucket in RGWGetACLs

Fix incorrect result when attempting to fetch an ACL for a
bucket that does not exist.

This change leads to a NoSuchBucket error result when an ACL is
requested from a non-existent bucket.  A NoSuchKey error result
iss already seen when the target was an object.

Fixes: https://tracker.ceph.com/issues/38116

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit 078981c)

Conflicts:
	src/rgw/rgw_op.cc : Resolved in verify_permission

src/rgw/rgw_op.cc

@@ -4625,6 +4625,9 @@ int RGWGetACLs::verify_permission()
 				    rgw::IAM::s3GetObjectAcl :
 				    rgw::IAM::s3GetObjectVersionAcl);
   } else {
+    if (!s->bucket_exists) {
+      return -ERR_NO_SUCH_BUCKET;
+    }
     perm = verify_bucket_permission(s, rgw::IAM::s3GetBucketAcl);
   }
   if (!perm)