LU-15121 llite: skip request slot for lmv_revalidate_slaves()

LU-15121 llite: skip request slot for lmv_revalidate_slaves()

Some syscalls need lmv_revalidate_slaves(). It requires
second lock enqueue and the it can be blocked by
lack of RPC slots.

Don't acquire rpc slot for second lock enqueue.

Lustre-change: https://review.whamcloud.com/45275
Lustre-commit: 7e781c605c4189ea1f4b0a343863280ebeb237d4

Change-Id: Ida23c648c2bd169c4d238543731796232aa490dc
HPE-bug-id: LUS-8416
Signed-off-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Vitaly Fertman <c17818@cray.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46106
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>