tracing: Add snapshot refcount

tracing: Add snapshot refcount

When a ring-buffer is memory mapped by user-space, no trace or
ring-buffer swap is possible. This means the snapshot feature is
mutually exclusive with the memory mapping. Having a refcount on
snapshot users will help to know if a mapping is possible or not.

Instead of relying on the global trace_types_lock, a new spinlock is
introduced to serialize accesses to trace_array->snapshot. This intends
to allow access to that variable in a context where the mmap lock is
already held.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]
Signed-off-by: Vincent Donnefort <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>