padata: Convert from atomic_t to refcount_t on parallel_data->refcnt

[ Upstream commit d5ee8e75 ]

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Acked-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Stable-dep-of: 7ddc21e3 ("padata: Fix refcnt handling in padata_free_shell()")
Signed-off-by: Sasha Levin <sashal@kernel.org>