libceph: convert ceph_pagelist.refcnt from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/pagelist.c

@@ -16,7 +16,7 @@ static void ceph_pagelist_unmap_tail(struct ceph_pagelist *pl)
 
 void ceph_pagelist_release(struct ceph_pagelist *pl)
 {
-	if (!atomic_dec_and_test(&pl->refcnt))
+	if (!refcount_dec_and_test(&pl->refcnt))
 		return;
 	ceph_pagelist_unmap_tail(pl);
 	while (!list_empty(&pl->head)) {