migration/rdma: prevent from double free the same mr

migration/rdma: prevent from double free the same mr

backtrace:
'0x00007ffff5f44ec2 in __ibv_dereg_mr_1_1 (mr=0x7fff1007d390) at /home/lizhijian/rdma-core/libibverbs/verbs.c:478
478             void *addr              = mr->addr;
(gdb) bt
 #0  0x00007ffff5f44ec2 in __ibv_dereg_mr_1_1 (mr=0x7fff1007d390) at /home/lizhijian/rdma-core/libibverbs/verbs.c:478
 #1  0x0000555555891fcc in rdma_delete_block (block=<optimized out>, rdma=0x7fff38176010) at ../migration/rdma.c:691
 #2  qemu_rdma_cleanup (rdma=0x7fff38176010) at ../migration/rdma.c:2365
 #3  0x00005555558925b0 in qio_channel_rdma_close_rcu (rcu=0x555556b8b6c0) at ../migration/rdma.c:3073
 #4  0x0000555555d652a3 in call_rcu_thread (opaque=opaque@entry=0x0) at ../util/rcu.c:281
 #5  0x0000555555d5edf9 in qemu_thread_start (args=0x7fffe88bb4d0) at ../util/qemu-thread-posix.c:541
 #6  0x00007ffff54c73f9 in start_thread () at /lib64/libpthread.so.0
 #7  0x00007ffff53f3b03 in clone () at /lib64/libc.so.6 '

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Message-Id: <20210708144521.1959614-1-lizhijian@cn.fujitsu.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>