We hit LU-9372 OOM problems, and after applying

    LU-9372 ptlrpc: allow to limit number of service's rqbds

    we found two problems:

    1)Since 0 is a reserved value for @srv_nrqbds_max which

    means unlimited value, procfs write interface should support

    this value, otherwise, there is no way to change default behavior

    back.

    2)the check in ptlrpc_lprocfs_req_history_max_seq_write() was broken

    after this patch, the following check will always succeed if @srv_nrqbds_max

    is kept as default value 0:

    val > svc->srv_nrqbds_max/2