SUNRPC: Convert rpc_client refcount to use refcount_t

[ Upstream commit 71d3d0eb ]

There are now tools in the refcount library that allow us to convert the
client shutdown code.

Reported-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Stable-dep-of: 4db9ad82 ("sunrpc: clear XPRT_SOCK_UPD_TIMEOUT when reset transport")
Signed-off-by: Sasha Levin <sashal@kernel.org>