LU-14959 ldlm: Check return value of ldlm_resource_get()

LU-14959 ldlm: Check return value of ldlm_resource_get()

Fix the comment to properly indicate it returns ERR_PTR on
error and fix osc_req_attr_set() and mdc_get_lock_handle()
to actually check the return value before passing it on and
causing an unintended crash.

Change-Id: Ib85a62140a39744e85989c9a9c8aa2ed771d70d1
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44738
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>