Limit re-opening WAL for truncating to current size

[pdillinger]

Currently, we have code that calls ReopenWritableFile on the latest WAL file on recovery:

https://github.com/facebook/rocksdb/blob/9.0.fb/db/db_impl/db_impl_open.cc#L1555-L1566

with the purpose of freeing any fallocated space that might be leftover. This is unnecessary on filesystems that don't support fallocate, or maybe don't even support re-opening a file for write after it has been closed.