nsproxy: Convert nsproxy.count to refcount_t

nsproxy: Convert nsproxy.count to refcount_t

atomic_t variables are currently used to implement reference counters
with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows and
underflows. This is important since overflows and underflows can lead
to use-after-free situation and be exploitable.

The variable nsproxy.count is used as pure reference counter. Convert it
to refcount_t and fix up the operations.

**Important note for maintainers:

Some functions from refcount_t API defined in refcount.h have different
memory ordering guarantees than their atomic counterparts. Please check
Documentation/core-api/refcount-vs-atomic.rst for more information.

Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in some
rare cases it might matter. Please double check that you don't have
some undocumented memory guarantees for this variable usage.

For the nsproxy.count it might make a difference in following places:
 - put_nsproxy() and switch_task_namespaces(): decrement in
   refcount_dec_and_test() only provides RELEASE ordering and ACQUIRE
   ordering on success vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230818041327.gonna.210-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>