netback: take net_schedule_list_lock when removing entry from net_schedule_list

netback: take net_schedule_list_lock when removing entry from net_schedule_list

There is a race in net_tx_build_mops between checking if
net_schedule_list is empty and actually dequeuing the first entry on
the list. If another thread dequeues the only entry on the list during
this window we crash because list_first_entry expects a non-empty
list. Therefore after the initial lock free check for an empty list
check again with the lock held before dequeueing the entry.

Based on a patch by Tomasz Wroblewski.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@novell.com>