LU-10175 ldlm: IBITS lock convert instead of cancel

LU-10175 ldlm: IBITS lock convert instead of cancel

For IBITS lock it is possible to drop just conflicting
bits and keep lock itself instead of cancelling it.
Lock convert is only bits downgrade on client and then
on server.
Patch implements lock convert during blocking AST.

Test-Parameters: mdssizegb=20 testlist=sanity-dom,dom-performance,racer,racer,racer
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: Id073f14ec3283479a9a48fd80090ecd3b0967b6d
Reviewed-on: https://review.whamcloud.com/30202
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>