LU-13593 ptlrpc: fix growing message buffer

LU-13593 ptlrpc: fix growing message buffer

In case some buffers need to be moved because of segment growth
from req_capsule_server_grow(), just set buflen to old length
before actually calling lustre_grow_msg().

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I6707927a0f24c0637dbc79aa91788122a84ab8c4
Reviewed-on: https://review.whamcloud.com/38701
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>