Pass the inferior argument available in thread_db_inferior_created, and
use it to do most things requiring the inferior.
check_pid_namespace_match is not completely decoupled from the current
inferior yet, there are hidden references behind target_can_run, for
example. But I think this is still a good step forward.
gdb/ChangeLog:
* linux-thread-db.c (check_pid_namespace_match): Add inferior
parameter and use it.
(thread_db_inferior_created): Pass inferior argument.