cpucontrol: check for the supposed firmware file type and skip

non-regular entry.

do_update() skips entries with DT_DIR type.  This does not eliminate
other entries that might exist in the directory.  More, since NFS
might fill d_type with DT_UNKNOWN, dot and dotdot entries are not
skipped, then mmap(2) call failed for them when update microcode files
are located on NFS.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

usr.sbin/cpucontrol/cpucontrol.c

@@ -369,6 +369,8 @@ try_a_fw_image(const char *dev_path, int devfd, int fwdfd, const char *dpath,
 		rc = 0;
 		goto out;
 	}
+	if (!S_ISREG(st.st_mode))
+		goto out;
 	if (st.st_size <= 0) {
 		WARN(0, "%s: empty", fw_path);
 		goto out;