BACKPORT: fs: align IOCB_* flags with RWF_* flags

We have a set of flags that are shared between the two and inherired
in kiocb_set_rw_flags(), but we check and set these individually.
Reorder the IOCB flags so that the bottom part of the space is synced
with the RWF flag space, and then we can do them all in one mask and
set operation.

The only exception is RWF_SYNC, which needs to mark IOCB_SYNC and
IOCB_DSYNC. Do that one separately.

This shaves 15 bytes of text from kiocb_set_rw_flags() for me.

(cherry picked from commit ce71bfea207b4d7c21d36f24ec37618ffcea1da8)
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Change-Id: Ib6316ae5cb3f8a14fabef5492e79783c9e6d3c4d
Signed-off-by: Alessio Balsini <balsini@google.com>