memtx: fix integer overflow in memtx index statistics

This may result in invalid statistics if there's more than 4 GB
allocated for index extents.

Fixes commit a75f4b7e ("memtx: introduce read view statistics").
Follow-up #8501
Coverity report 1537026, 1537027.

NO_DOC=bug fix
NO_CHANGELOG=unreleased
NO_TEST=complicated; checked by coverity