Fix the following race:

1. Process p1 is currently being swapped in.
2. Process p2 calls linux_ptrace(PTRACE_GETFPXREGS, p1_pid, ...)
3. After acquiring a reference to FIRST_THREAD_IN_PROC(p1), p2 blocks in faultin() while p1 finishes being swapped in. This means p2 won't get back the lock on p1 until after p1's threads are runnable.
4. After p1 is swapped in, the first thread in p1 exits.
5. p2 now uses its dangling reference to p1's first thread.