tracing/user_events: Move pages/locks into groups to prepare for namespaces

In order to enable namespaces or any sort of isolation within
user_events the register lock and pages need to be broken up into
groups. Each event and file now has a group pointer which stores the
actual pages to map, lookup data and synchronization objects.

This only enables a single group that maps to init_user_ns, as IMA
namespace has done. This enables user_events to start the work of
supporting namespaces by walking the namespaces up to the init_user_ns.
Future patches will address other user namespaces and will align to the
approaches the IMA namespace uses.

Link: https://lore.kernel.org/linux-kernel/[email protected]/#t
Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Beau Belgrave <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: beaubelgrave