arm64: log vm_fault error for data_abort

Summary:
Log the vm_fault() error in the data_abort panic so it is easier to
find the reason vm_fault() failed (e.g., invalid address).

Reviewed by:	andrew
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D30362

sys/arm64/arm64/trap.c

@@ -334,7 +334,8 @@ data_abort(struct thread *td, struct trapframe *frame, uint64_t esr,
 					return;
 			}
 #endif
-			panic("vm_fault failed: %lx", frame->tf_elr);
+			panic("vm_fault failed: %lx error %d",
+			    frame->tf_elr, error);
 		}
 	}