Remove auto-detection of optimal buffer size
Remove auto-detection of optimal buffer size

Since we don't pass zero anymore, this code is never called. Replace
it with 'zero means hardcoded initial value', since that hardcoded
value is what we were already passing, and change all the callers to
pass in zero (as 'auto-compute' again) instead, for simplicity.

iustin/pyxattr,iustin/pyxattr