Widen uk_pgoff, the slab header offset field.

16 bits is only wide enough for kegs with an item size of up to 64KB.
At that size or larger, slab headers are typically offpage because the
item size is a multiple of the page size, but there is no requirement
that this be the case.

We can widen the field without affecting the layout of struct uma_keg
since the removal of uk_slabsize in r315077 left an adjacent hole.

PR: 218911
MFC after: 2 weeks