fs: switch f_iocb_flags and f_ra

fs: switch f_iocb_flags and f_ra

Now that we shrank struct file by 24 bytes we still have a 4 byte hole.
If we move struct file_ra_state into the union and f_iocb_flags out of
the union we close that whole and bring down struct file to 192 bytes.
Which means struct file is 3 cachelines and we managed to shrink it by
40 bytes this cycle.

I've tried to audit all codepaths that use f_ra and none of them seem to
rely on it in file->f_op->release() and never have since commit
1da177e4c3f4 ("Linux-2.6.12-rc2").

Link: https://lore.kernel.org/r/20240823-luftdicht-berappen-d69a2166a0db@brauner
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>