Introduce enum flag_mode for setting and clearing flags.

Introduce enum flag_mode for setting and clearing flags.

We currently use '1' to indicate that a flag (writemostly or failfast)
needs to be set, and '2' to indicate that it needs to be cleared.

Using magic number like this is not a best-practice.

So replaced them with values from a enum.

No functional change.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>