powerpc/vnic: Extend "failover pending" window

powerpc/vnic: Extend "failover pending" window

Commit 5a18e1e0c193b introduced the 'failover_pending' state to track
the "failover pending window" - where we wait for the partner to become
ready (after a transport event) before actually attempting to failover.
i.e window is between following two events:

        a. we get a transport event due to a FAILOVER

        b. later, we get CRQ_INITIALIZED indicating the partner is
           ready  at which point we schedule a FAILOVER reset.

and ->failover_pending is true during this window.

If during this window, we attempt to open (or close) a device, we pretend
that the operation succeded and let the FAILOVER reset path complete the
operation.

This is fine, except if the transport event ("a" above) occurs during the
open and after open has already checked whether a failover is pending. If
that happens, we fail the open, which can cause the boot scripts to leave
the interface down requiring administrator to manually bring up the device.

This fix "extends" the failover pending window till we are _actually_
ready to perform the failover reset (i.e until after we get the RTNL
lock). Since open() holds the RTNL lock, we can be sure that we either
finish the open or if the open() fails due to the failover pending window,
we can again pretend that open is done and let the failover complete it.

We could try and block the open until failover is completed but a) that
could still timeout the application and b) Existing code "pretends" that
failover occurred "just after" open succeeded, so marks the open successful
and lets the failover complete the open. So, mark the open successful even
if the transport event occurs before we actually start the open.

Fixes: 5a18e1e0c193 ("ibmvnic: Fix failover case for non-redundant configuration")
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Acked-by: Dany Madden <drt@linux.ibm.com>
Link: https://lore.kernel.org/r/20201030170711.1562994-1-sukadev@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>