LU-3470 llog: fix return value of llog_alloc_handle

LU-3470 llog: fix return value of llog_alloc_handle

llog_open() calls llog_alloc_handle() taking NULL as the error return
value. But llog_alloc_handle() returns ERR_PTR(-ENOMEM) instead when
error.

Signed-off-by: Li Xi <pkuelelixi@gmail.com>
Change-Id: Id0e9d94107776f65bf6fdaba21988bab1882d3b3
Reviewed-on: http://review.whamcloud.com/6644
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>