LU-1403 ucred: ucred cleanup

LU-1403 ucred: ucred cleanup

Add ucred_check() and ucred_assert() methods to ensure ucred is
installed and/or initialised everywhere.

As echo client may be sitting on mdd it needs to initialize md_ucred
too. As echo client may be sitting on osc without mdd module loaded,
make md_ucred a common key, move it to lu_object.

Reviewed-by: Alexander Zarochentsev <Alexander_Zarochentsev@xyratex.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Xyratex-bug-id: MRP-484
Change-Id: I9c4aaa31148ff10e4a609393a0cf018cdf731d9c
Reviewed-on: http://review.whamcloud.com/2733
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>