LU-16454 mdt: Add a per-MDT "max_mod_rpcs_in_flight"

LU-16454 mdt: Add a per-MDT "max_mod_rpcs_in_flight"

Value max_mod_rpcs_per_client doesn't define a static number of
slots for the per-client replies or anything, and the only
thing it is used for is to pass the limit to the client. For the
same reason, there also doesn't appear to be a particularly hard
limitation why the client cannot change and exceed the
server-provided parameter, except to avoid overloading the server
with too many RPCs at once, but that may also be true of the
current limit with a larger number of clients, no different than
"max_rpcs_in_flight".

This fix adds a tunable parameter "max_mod_rpcs_in_flight" per MDT
to lustre/mdt/mdt_lproc.c so that it can be set
with "lctl set_param" at runtime. The max_mod_rpcs_per_client global
setting is marked "deprecated" but is still used as the default
value when creating an MDT.

Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I27cfcb68e1a534e80e6a2dbf2e1affc430803b49
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49749
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>