SUNRPC: The RDMA back channel mustn't disappear while requests are outstanding

SUNRPC: The RDMA back channel mustn't disappear while requests are outstanding

BugLink: https://bugs.launchpad.net/bugs/1852338
[ Upstream commit 9edb455e6797bb50aa38ef71e62668966065ede8 ]

If there are RDMA back channel requests being processed by the
server threads, then we should hold a reference to the transport
to ensure it doesn't get freed from underneath us.

Reported-by: Neil Brown <neilb@suse.de>
Fixes: 63cae47005af ("xprtrdma: Handle incoming backward direction RPC calls")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>