LU-12347 llite: do not take mod rpc slot for getxattr

LU-12347 llite: do not take mod rpc slot for getxattr

The following scenario may lead to client eviction:
clientA                clientB                  MDS
threadA1: write to file F1, get
and hold DoM MDC LDLM lock L1:
   ->cl_io_loop()
    ->cl_io_lock()
     :
     ->mdc_lock_granted()
      ->lock->l_writers++
      [hold ref until write done]

threadA2-A8: create files F2-F8:
   ->ll_file_open()
    ->mdc_enqueue_base()
     ->ldlm_cli_enqueue()
      ->ptlrpc_get_mod_rpc_slot()
      ->ptlrpc_queue_wait()
      [hold RPC slot until create done]

                                              OST(s) in recovery.
                                              MDS waiting on OST(s) to
                                              precreate new objects.

threadA1:
   -> cl_io_start()
    -> __generic_file_aio_write()
     -> file_remove_suid()
      -> ll_xattr_cache_refill()
       -> mdc_xattr_common()
        -> ptlrpc_get_mod_rpc_slot()
        [blocked waiting for RPC slot]

                       threadB1: write file F1,
		       enqueue DoM MDC lock L1

                                              MDS sends blocking AST
                                              to clientA for lock L1

ldlm_threadA3: cannot cancel busy lock L1:
   -> ldlm_handle_bl_callback()
   ["Lock L1 referenced, will be cancelled later"]

                                              MDS evicts clientA for
                                              not cancelling lock L1

threadA1: never completes write:
  ->cl_io_end()
   ->cl_io_unlock()
    ->osc_lock_cancel()
     ->lock->l_writers--;

The fix is to add IT_GETXATTR to list of operations which do not
need mod rpc slot.

Tests to illustrate the issue is added.

wait_for_function(): total sleep time (wait) is to be equal to max
when 1 is returned.

Signed-off-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
HPE-bug-id: LUS-7271
Change-Id: I1b80677df084bda141b9ac58a78b765bd0b14a41
Reviewed-on: https://review.whamcloud.com/44151
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>