staging/lustre/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.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3470
Lustre-change: http://review.whamcloud.com/6644


Signed-off-by: Li Xi <pkuelelixi@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>