afs: Use refcount_t rather than atomic_t

[ Upstream commit c56f9ec8 ]

Use refcount_t rather than atomic_t in afs to make use of the count
checking facilities provided.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/165911277768.3745403.423349776836296452.stgit@warthog.procyon.org.uk/

 # v1
Stable-dep-of: 9a6b294a ("afs: Fix use-after-free due to get/remove race in volume tree")
Signed-off-by: Sasha Levin <sashal@kernel.org>