performance: Do not use O_DIRCET

Benchmarks show that O_DIRECT cases x10 performance degradation. Issue still
need to be checked, but for now, no O_DIRECT for you.

Check using:
  $ dd if=/dev/zero of=~funex/zero count=10000 bs=100000

Signed-off-by: Shachar Sharon <shachars@xxxxxx>