Don't write to inferior_ptid in inf-ptrace.c

Don't write to inferior_ptid in inf-ptrace.c

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* inf-ptrace.c (inf_ptrace_target::create_inferior): Switch to the
	added thread.
	(inf_ptrace_target::attach): Don't write to inferior_ptid.  Switch
	to the added thread.
	(inf_ptrace_target::detach_success): Use switch_to_no_thread
	instead of writing to inferior_ptid.