coredump: use fstatvfs to check the available space

Given that we already have the file descriptor opened for writing, it
would make sense to call fstatvfs with that file descriptor rather than
statvfs with the directory path that was used to open that descriptor.

src/coredump/coredump.c

@@ -597,7 +597,7 @@ static int save_external_coredump(
                 /* tmpfs might get full quickly, so check the available space too.
                  * But don't worry about errors here, failing to access the storage
                  * location will be better logged when writing to it. */
-                if (statvfs("/var/lib/systemd/coredump/", &sv) >= 0)
+                if (fstatvfs(fd, &sv) >= 0)
                         max_size = MIN((uint64_t)sv.f_frsize * (uint64_t)sv.f_bfree, max_size);
 
                 log_debug("Limiting core file size to %" PRIu64 " bytes due to cgroup memory limits.", max_size);