Bluetooth: att: Fix deadlock on meta data context allocation

This fixes deadlock that happened waiting for meta data in system
workqueue.
The meta data always get freed in the system workqueue,
so if we're in the same workqueue but there are no immediate
contexts available, there's no chance we'll get one by waiting.

Fixes: #53455
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>