swapon: extend the region where the swap vnode is locked

to cover VOP_GETATTR() call in sys_swapon(). Move locking from inside
swapongeom() and swaponvp() into sys_swapon().

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