tree-wide: port everything over to fflush_and_check()

Some places invoked fflush() directly with their own manual error
checking, let's unify all that by using fflush_and_check().

This also unifies the general error paths of fflush()+rename() file
writers.

src/network/networkd-link.c

@@ -2388,14 +2388,13 @@ int link_save(Link *link) {
         }
 
         return 0;
-fail:
-        log_link_error_errno(link, r, "Failed to save link data to %s: %m", link->state_file);
-        (void) unlink(link->state_file);
 
+fail:
+        (void) unlink(link->state_file);
         if (temp_path)
                 (void) unlink(temp_path);
 
-        return r;
+        return log_link_error_errno(link, r, "Failed to save link data to %s: %m", link->state_file);
 }
 
 static const char* const link_state_table[_LINK_STATE_MAX] = {

src/network/networkd-manager.c

@@ -818,10 +818,10 @@ int manager_save(Manager *m) {
         return 0;
 
 fail:
-        log_error_errno(r, "Failed to save network state to %s: %m", m->state_file);
-        unlink(m->state_file);
-        unlink(temp_path);
-        return r;
+        (void) unlink(m->state_file);
+        (void) unlink(temp_path);
+
+        return log_error_errno(r, "Failed to save network state to %s: %m", m->state_file);
 }
 
 int manager_address_pool_acquire(Manager *m, int family, unsigned prefixlen, union in_addr_union *found) {