refcount_t: Introduce a special purpose refcount type

Provide refcount_t, an atomic_t like primitive built just for
refcounting.

It provides saturation semantics such that overflow becomes impossible
and thereby 'spurious' use-after-free is avoided.

Change-Id: Id360e29bb9c19af8672726539d0d16c0f6d2be3f
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Git-commit: f405df5de3170c00e5c54f8b7cf4766044a032ba
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>