SUNRPC: Fix a backchannel deadlock

xprt_alloc_bc_request() cannot call xprt_free_bc_request() without
deadlocking, since it already holds the xprt->bc_pa_lock.

Reported-by: Chuck Lever <chuck.lever@oracle.com>
Fixes: 0d2a970d ("SUNRPC: Fix a backchannel race")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>