LU-9066 ldlm: don't evict client on umount if AST fails

LU-9066 ldlm: don't evict client on umount if AST fails

When server is being stopped the AST send is denied and
that AST failure will lead to the unexpected client
eviction.
Patch adds extra checks in ldlm_handle_ast_error() to
distinguish that type of error and proceed without
eviction.

Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: I57c7f417376ac64c8df4c6d5f68bea5083c0d22f
Reviewed-on: https://review.whamcloud.com/23921
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>