snapshotctl: fsync after writing every 1MB buffer

Sync writes after every 1MB instead of flushing at the end.

Bug: 299011882
Test: Boot device off snapshots
Change-Id: If91168ec92c2b2995bdf296ea1c7d4c261b12411
Signed-off-by: Akilesh Kailash <akailash@google.com>