FROMLIST: fs: Generic 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.

Reduce the OverlayFS code by making the flag conversion function generic
and reusable.

Bug: 168023149
Link: https://lore.kernel.org/lkml/20210125153057.3623715-2-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Change-Id: I74aefeafd6ebbda2fbabee9024474dfe4cc6c2a7
Signed-off-by: Alessio Balsini <balsini@google.com>