xprtrdma: rpcrdma_mr_pop() already does list_del_init()

The rpcrdma_mr_pop() earlier in the function has already cleared
out mr_list, so it must not be done again in the error path.

Fixes: 84756894

 ("xprtrdma: Remove fr_state")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>