fs: ext4: fsync: optimize double-fsync() a bunch

There are cases where EXT4 is a bit too conservative sending barriers down to
the disk; there are cases where the transaction in progress is not the one
that sent the barrier (in other words: the fsync is for a file for which the
IO happened more time ago and all data was already sent to the disk).

For that case, a more performing tradeoff can be made on SSD devices (which
have the ability to flush their dram caches in a hurry on a power fail event)
where the barrier gets sent to the disk, but we don't need to wait for the
barrier to complete. Any consecutive IO will block on the barrier correctly.

Signed-off-by: Diab Neiroukh <lazerl0rd@thezest.dev>
Signed-off-by: Yaroslav Furman <yaro330@gmail.com>