Simplify extent_quantize().
Simplify extent_quantize().

2cdf07aba971d1e21edc203e7d4073b6ce8e72b9 (Fix extent_quantize() to
handle greater-than-huge-size extents.) solved a non-problem; the
expression passed in to index2size() was never too large.  However the
expression could in principle underflow, so fix the actual (latent) bug
and remove unnecessary complexity.

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc