object-file: rename files transaction fsync function

object-file: rename files transaction fsync function

When writing an object to a "files" ODB transaction, a full hardware
flush is not initially performed during the fsync in
`fsync_loose_object_transaction()` and instead delayed until the
transaction is later committed.

To be more consistent with other "files" ODB transaction helpers, rename
the function to `odb_transaction_files_fsync()` accordingly. The
conditional in the helper is also slightly restructured to improve
clarity to readers.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>