LU-10603 ptlrpc: export req_buffers_max via procfs

LU-10603 ptlrpc: export req_buffers_max via procfs

after LU-9372 gcc7 complains:
lustre/ptlrpc/lproc_ptlrpc.c:382:16: error: ‘ptlrpc_lprocfs_req_buffers_max_fops’ defined but not used [-Werror=unused-const-variable=]
 LPROC_SEQ_FOPS(ptlrpc_lprocfs_req_buffers_max);
                 ^

Change-Id: Ie4806b79d104c7ea9aa34b6a8a280587fccef689
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/31162
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>