gdb: pass inferior to check_pid_namespace_match

gdb: pass inferior to check_pid_namespace_match

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.

Change-Id: Ib768b14fc61dcf115fe13f776691f2c2f36e0679