The in-kernel CTF parser caches the result of its first attempt to parse

CTF data from a module.  On subsequent attempts to retrieve CTF data for
a module, return an error if there no CTF data.

This fixes a panic if you try to enable fbt probes on a module with CTF
data twice.

Submitted by:	Paul Ambrose (ambrosehua AT gmail DOT com)
MFC after:	3 days

sys/kern/kern_ctf.c

@@ -90,7 +90,7 @@ link_elf_ctf_get(linker_file_t lf, linker_ctf_t *lc)
 	 * ctfcnt to -1. See below.
 	 */
 	if (ef->ctfcnt < 0)
-		return (0);
+		return (EFTYPE);
 
 	/* Now check if we've already loaded the CTF data.. */
 	if (ef->ctfcnt > 0) {