LU-5002 flock: accept EDEADLK in sanityn test_74

In flocks_test.c t4() acquiring the second flock may legitimately fail
and return -EDEADLK. Handle this and ensure that errors encountered by
the child process are returned by the parent.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I55172add1412377b1c7125efa4177702d8bf1834
Reviewed-on: http://review.whamcloud.com/10208
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>