SUNRPC: xprt_complete_bc_request must also decrement the free slot count

Calling xprt_complete_bc_request() effectively causes the slot to be allocated,
so it needs to decrement the backchannel free slot count as well.

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