utils: Add ref_cur() to retrieve the current value of a reference counter

utils: Add ref_cur() to retrieve the current value of a reference counter

On many architectures it is safe to read the value directly (those
using cache coherency protocols, and with atomic loads for 32-bit
values) but it is not if that's not the case or if we ever decide to
make refcount_t 64-bit (load not atomic on x86).

So make sure the operation is actually atomic and that users do not
have to care about the size of refcount_t.