uncrypt: remove O_SYNC to avoid time-out failures

This patch removes costly O_SYNC flag for encrypted block device.
After writing whole decrypted blocks, fsync should guarantee their consistency
from further power failures.
This patch reduces the elapsed time significantly consumed by upgrading packages
on an encrypted partition, so that it could avoid another time-out failures too.

Change-Id: I1fb9022c83ecc00bad09d107fc87a6a09babb0ec
Signed-off-by: Jaegeuk Kim <jaegeuk@motorola.com>