net/mlx4_core: mlx4_init_slave() shouldn't access comm channel before PF is ready

Currently, the PF call to pci_enable_sriov from the PF probe function
stalls for 10 seconds times the number of VFs probed on the host. This
happens because the way for such VFs to determine of the PF
initialization finished, is by attempting to issue reset on the
comm-channel and get timeout (after 10s).

The PF probe function is called from a kenernel workqueue, and therefore
during that time, rcu lock is being held and kernel's workqueue is
stalled. This blocks other processes that try to use the workqueue
or rcu lock.  For example, interface renaming which is calling
rcu_synchronize is blocked, and timedout by systemd.

Changed mlx4_init_slave() to allow VF probed on the host to immediatly
detect that the PF is not ready, and return EPROBE_DEFER instantly.

Only when the PF finishes the initialization, allow such VFs to
access the comm channel.

This issue and fix are relevant only for probed VFs on the hypervisor,
there is no way to pass this information to a VM until comm channel is
ready, so in a VM, if PF is not ready, the first command will be timedout
after 10 seconds and return EPROBE_DEFER.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>