hwpmc: release a PMC whose ID could not be copied out

If copyout of the newly allocated PMC ID fails, the caller cannot use or
release the PMC. Release it in the kernel.

Reviewed by:	mhorne
MFC after:	3 days
Sponsored by:	Advanced Micro Devices, Inc.
Differential Revision:	https://reviews.freebsd.org/D60562

sys/dev/hwpmc/hwpmc_mod.c

@@ -4367,6 +4367,12 @@ pmc_syscall_handler(struct thread *td, void *syscall_args)
 			break;
 
 		error = copyout(&pa, arg, sizeof(pa));
+		/*
+		 * The caller never learns the ID of an unpublished PMC; do
+		 * not leave it (and the row it holds) behind.
+		 */
+		if (error != 0)
+			(void)pmc_do_op_pmcrelease(pa.pm_pmcid);
 	}
 	break;