New helper to be used instead of direct checks for IOCB_DSYNC:
iocb_is_dsync(iocb).  Checks converted, which allows to avoid
the IS_SYNC(iocb->ki_filp->f_mapping->host) part (4 cache lines)
from iocb_flags() - it's checked in iocb_is_dsync() instead
Reviewed-by: NChristian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>