staging/lustre/osc: use global osc_rq_pool to reduce memory usage

The per-osc request pools consume a lot of memory if there are
hundreds of OSCs on one client. This will be a critical problem
if the client doesn't have sufficient memory for both OSCs and
applications.

This patch replaces per-osc request pools with a global pool
osc_rq_pool. The total memory usage is 5MB by default. And it
can be set by a module parameter of OSC:
"options osc osc_reqpool_mem_max=POOL_SIZE". The unit of POOL_SIZE
is MB. If cl_max_rpcs_in_flight is the same for all OSCs, the
memory usage of the OSC pool can be calculated as:
Min(POOL_SIZE * 1M,
    (cl_max_rpcs_in_flight + 2) * OSC number * OST_MAXREQSIZE)

Also, this patch changes the allocation logic of OSC write requests.
The allocation from osc_rq_pool will only be tried after normal
allocation failed.

Signed-off-by: Wu Libin <lwu@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Li Xi <lixi@ddn.com>
Reviewed-on: http://review.whamcloud.com/15422
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6770


Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>