LU-17076 nrs: wait for RCU completion

LU-17076 nrs: wait for RCU completion

before we destroy the slab holding the objects scheduled
for a release via RCU.

    BUG: unable to handle kernel paging request at 000000046474e5c6
    Oops: 0000 [#1] SMP
    CPU: 0 PID: 9 Comm: ksoftirqd/0  4.18.0 #3
    RIP: 0010:kmem_cache_free+0xe3/0x170
    Call Trace:
     rcu_core+0x27a/0x770
     __do_softirq+0xc2/0x44d
     run_ksoftirqd+0x35/0x50
     smpboot_thread_fn+0xb8/0x160
     kthread+0x14a/0x170

Fixes: 42bf5f78ba ("LU-8130 nrs: convert NRS ORR/TRR to rhashtable")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ia4b1cf6f190f17c3b85548fcb6876be72822cdd2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52515
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>