Invoke ext_free function when freeing an unmapped mbuf.

Fix a mis-merge when extracting the unmapped mbuf changes from
Netflix's in-kernel TLS changes where the call to the function that
freed the backing pages from an unmapped mbuf was missed.

Sponsored by:	Chelsio Communications

sys/kern/kern_mbuf.c

@@ -1282,6 +1282,9 @@ mb_free_ext(struct mbuf *m)
 			uma_zfree(zone_mbuf, mref);
 			break;
 		case EXT_PGS:
+			KASSERT(mref->m_ext.ext_free != NULL,
+			    ("%s: ext_free not set", __func__));
+			mref->m_ext.ext_free(mref);
 			uma_zfree(zone_extpgs, mref->m_ext.ext_pgs);
 			uma_zfree(zone_mbuf, mref);
 			break;