LU-13365 ldlm: check slv and limit before updating

slv and limit do not change for most of time,
ldlm_cli_update_pool() could be called for each RPC reply,
try hold read lock to check firstly could avoid heavy write
lock in hot path.

Change-Id: I4ef33132a463b3bac23863c09a2652f2d2782aae
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37969
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>