LU-12542 ldlm: don't access l_resource when not locked.

LU-12542 ldlm: don't access l_resource when not locked.

lock->l_resource can (sometimes) change when the resource
isn't locked.
So dereferencing lock->l_resource and then locking the
resource looks wrong.
As lock_res_and_lock() returns the locked resource, this
code can easily be more obviously correct by using
that return value.

Change-Id: Iced0bf1af4fa8ddedffa817e00f1c6a02b035d76
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/35484
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>