perf: protect group_leader from races that cause ctx double-free

When moving a group_leader perf event from a software-context
to a hardware-context, there's a race in checking and
updating that context. The existing locking solution
doesn't work; note that it tries to grab a lock inside
the group_leader's context object, which you can only
get at by going through a pointer that should be protected
from these races. To avoid that problem, and to produce
a simple solution, we can just use a lock per group_leader
to protect all checks on the group_leader's context.
The new lock is grabbed and released when no context locks
are held.

Bug: 30955111
Bug: 31095224
Change-Id: If37124c100ca6f4aa962559fba3bd5dbbec8e052
Git-repo: https://android.googlesource.com/kernel/msm


Git-commit: 5b87e00be9ca28ea32cab49b92c0386e4a91f730
[dcagle@codeaurora.org: Slight merge conflict resolution]
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>