gh-120754: Update estimated_size in C truncate (#121357)

Sometimes a large file is truncated (test_largefile). While
estimated_size is used as a estimate (the read will stil get the number
of bytes in the file), that it is much larger than the actual size of
data can result in a significant over allocation and sometimes lead to
a MemoryError / running out of memory.

This brings the C implementation to match the Python _pyio
implementation.