Remove usages of find_inferior calling not_stopped_callback

Remove usages of find_inferior calling not_stopped_callback

Replace with find_thread.  Writing a lambda inline in directly in the if
conditions would be a bit messy, so I chose to assign them to variables
instead.

gdb/gdbserver/ChangeLog:

	* linux-low.c (not_stopped_callback): Return bool, take filter
	argument directly.
	(linux_wait_for_event_filtered): Use find_thread.
	(linux_wait_1): Likewise.