fs: improve error performance of readvSync
Local benchmark:
confidence improvement accuracy (*) (**) (***)
fs/bench-readvSync.js n=100000 type='invalid' *** 123.02 % ±1.27% ±1.70% ±2.24%
fs/bench-readvSync.js n=100000 type='valid' 2.74 % ±2.88% ±3.84% ±5.01%