Kernel: Set new process name in `do_exec` before waiting for the tracer

While investigating why gdb is failing when it calls `PT_CONTINUE`
against Serenity I noticed that the names of the programs in the
System Monitor didn't make sense. They were seemingly stale.

After inspecting the kernel code, it became apparent that the sequence
occurs as follows:

    1. Debugger calls `fork()`
    2. The forked child calls `PT_TRACE_ME`
    3. The `PT_TRACE_ME` instructs the forked process to block in the
       kernel waiting for a signal from the tracer on the next call
       to `execve(..)`.
    4. Debugger waits for forked child to spawn and stop, and then it
       calls `PT_ATTACH` followed by `PT_CONTINUE` on the child.
    5. Currently the `PT_CONTINUE` fails because of some other yet to
       be found bug.
    6. The process name is set immediately AFTER we are woken up by
       the `PT_CONTINUE` which never happens in the case I'm debugging.

This chain of events leaves the process suspended, with the name  of
the original (forked) process instead of the name we inherit from
the `execve(..)` call.

To avoid such confusion in the future, we set the new name before we
block waiting for the tracer.

Kernel/Syscalls/execve.cpp

@@ -564,6 +564,9 @@ ErrorOr<void> Process::do_exec(NonnullRefPtr<OpenFileDescription> main_program_d
     //       and we don't want to deal with faults after this point.
     auto new_userspace_sp = TRY(make_userspace_context_for_main_thread(new_main_thread->regs(), *load_result.stack_region.unsafe_ptr(), m_arguments, m_environment, move(auxv)));
 
+    m_name = move(new_process_name);
+    new_main_thread->set_name(move(new_main_thread_name));
+
     if (wait_for_tracer_at_next_execve()) {
         // Make sure we release the ptrace lock here or the tracer will block forever.
         ptrace_locker.unlock();
@@ -583,9 +586,6 @@ ErrorOr<void> Process::do_exec(NonnullRefPtr<OpenFileDescription> main_program_d
 
     // NOTE: Be careful to not trigger any page faults below!
 
-    m_name = move(new_process_name);
-    new_main_thread->set_name(move(new_main_thread_name));
-
     {
         ProtectedDataMutationScope scope { *this };
         m_protected_values.promises = m_protected_values.execpromises.load();