If we fail to emulate a vm86 trap in kernel mode, then we use

vm86_trap() to return to the calling program directly.  vm86_trap()
doesn't return, thus it was never returning to trap() to release
Giant.  Thus, release Giant before calling vm86_trap().

sys/amd64/amd64/trap.c

@@ -467,6 +467,7 @@ trap(frame)
 					/*
 					 * returns to original process
 					 */
+					mtx_exit(&Giant, MTX_DEF);
 					vm86_trap((struct vm86frame *)&frame);
 				goto out;
 			}

sys/i386/i386/trap.c

@@ -467,6 +467,7 @@ trap(frame)
 					/*
 					 * returns to original process
 					 */
+					mtx_exit(&Giant, MTX_DEF);
 					vm86_trap((struct vm86frame *)&frame);
 				goto out;
 			}

sys/kern/subr_trap.c

@@ -467,6 +467,7 @@ trap(frame)
 					/*
 					 * returns to original process
 					 */
+					mtx_exit(&Giant, MTX_DEF);
 					vm86_trap((struct vm86frame *)&frame);
 				goto out;
 			}