ovl: use simpler function to convert iocb to rw flags

Overlayfs implements its own function to translate iocb flags into rw
flags, so that they can be passed into another vfs call.

With commit ce71bfea

 ("fs: align IOCB_* flags with RWF_* flags")
Jens created a 1:1 matching between the iocb flags and rw flags,
simplifying the conversion.

Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>