kernel: generic user_memcpy() should clear_ac after the fault handler returns

the AC flag in eflags/rflags, pushed in the iframe by the CPU, is kept intact after handling the exception, since the fault handler is run with the faulted iframe and does a simple jump. The AC flag would otherwise be set until the syscall returns to userland.

Change-Id: I24f763032ab98029dd162fb411e1541586451606
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4040
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

headers/private/kernel/arch/generic/user_memory.h

@@ -70,6 +70,7 @@ bool user_access(Function function)
 		clear_ac();
 		return true;
 	}
+	clear_ac();
 	return false;
 }