LU-10803 ptlrpc: fix req_buffers_max and req_history_max setting

LU-10803 ptlrpc: fix req_buffers_max and req_history_max setting

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

Change-Id: Ida0796fa500fe595e003accc11d20fdad5e60c03
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/31622
Tested-by: Jenkins
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>