interface: handle error during admin-up correctly

interface: handle error during admin-up correctly

Type: fix

In vnet_sw_interface_set_flags_helper(), the variable old_flags is set
to the original value of vnet_sw_interface_t.flags for an interface. If
an error occurs during the process of bringing an interface up, old_flags
is used to restore the original value.

Before the dev class or hw class admin_up_down_function can be called,
but after modifying vnet_sw_interface_t.flags to it's new value,
old_flags is set to the value of vnet_sw_interface_t.flags a second time.
This discards the original flags that were being preserved.

As a result, if an interface is being brought up and the dev class
or hw class function fails, at the end VPP believes that interface is up.
This can cause a crash if packets are routed through the interface
and some RX/TX initialization was not completed because of the error
while bringing the interface up.

Change-Id: Ica6b6bac13c24e88c4136bf084cd392e6217e7d9
Signed-off-by: Matthew Smith <mgsmith@netgate.com>