fellow_cache: enable segment checksum validation

I should have had done this a long time ago, no idea why I lost track of this
TODO item.

src/fellow_cache.c

@@ -4632,13 +4632,11 @@ fellow_cache_seg_check(struct fellow_cache_seg *fcs)
 
 	fds = fcs->disk_seg;
 	CHECK_OBJ_NOTNULL(fds, FELLOW_DISK_SEG_MAGIC);
-#ifdef TODO_READ_ERR
 	if ((fds->segnum % 16 == 0 && FC_INJ) ||
 	    fhcmp(fds->fht, fds->fh, fcs->alloc.ptr, fcs->u.fcs.len)) {
 		err = "segment checksum error";
 		to = FCS_READFAIL;
 	}
-#endif
 
 	fco = fcs->fco;
 	AZ(pthread_mutex_lock(&fco->mtx));