fd-tracker: remove use of VLA for unsuspendable_fd APIs

fd-tracker: remove use of VLA for unsuspendable_fd APIs

fd_tracker_open_unsuspendable_fd() and
fd_tracker_close_unsuspendable_fd() make use of variable-length arrays
to track unsuspendable entries to open/close. These uses of VLA can
easily be replaced by using dynamic allocations. This is not expected
to cause any performance problem as it is used in slow paths.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I4d4e95e23eb6df9a66663ecabad40162c321525b