swap_pager: lock vnode in swapdev_strategy()

VOP_STRATEGY() requires locked vnode. Note that we lock the swap vnode
while pages are busy, but this would only cause real LoR if pages belong
to the swap vnode, which must not be the case for correct use.

Reported and tested by: peterj
Reviewed by: markj
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33119