kvm tools, qcow: Don't fdatasync() L2 table writeout

kvm tools, qcow: Don't fdatasync() L2 table writeout

There's now now point in making sure new L2 tables actually hit the disk before
we write out data to clusters because they are not visible on-disk until
qcow_disk_flush() is called which flushes the L1 table.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>