Fix a logic error in the item size calculation for internal UMA zones.

Kegs for internal zones always keep the slab header in the slab itself.
Therefore, when determining the allocation size, we need to take the
slab header size into account.

Reported and tested by: ae, rakuco
Reviewed by: avg
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12342