swap_pager: swapoff detecting object death

In swap_pager_swapoff_object, the object is initially not dead, and
can only become dead while the object lock is not held. Move the test
for object-death so that it is right after the early loop-break that
happens after lock re-acquisition, and before the iterator is
re-initialized, which fails an assertion when the object is dead,
and not swap.

Addresses a problem produced in testing by @pho and diagnosed by @kib.

Reported by: pho
Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D47064