ap_tools/ap-check: handle get-attributes between pre and post event

Since mdevctl commit acf78c1ff6c9 it is now possible for the
get-attributes event to occur between a pre-define and post-define.
This is done in order to obtain the active attributes for the device
before writing them to the config file, and implies that the
get-attributes cannot re-obtain the file lock.  For other cases
where mdevctl calls get-attributes, the file lock is not already
held and must be obtained by ap-check before reading attributes from
active devices.
To solve this, let's use the knowledge that mdevctl is a single-threaded
tool and add a test to detect this scenario.  If the file lock is
already held by the parent during a get-attributes, don't attempt to
re-acquire it.

Reported-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

include/lib/ap.h

@@ -89,6 +89,7 @@ void ap_list_remove_all(struct util_list *list);
 /* Lock Functions */
 int ap_get_lock(void);
 int ap_get_lock_callout(void);
+int ap_try_lock_callout(void);
 int ap_release_lock(void);
 int ap_release_lock_callout(void);