ipv4: remove static flush_delay variable

flush delay is used as an external storage for net.ipv4.route.flush sysctl
entry. It is write-only.

The ctl_table->data for this entry is used once. Fix this case to point
to the stack to remove global variable. Do this to avoid additional
variable on struct net in the next patch.

Possible race (as it was before) accessing this local variable is removed
using flush_mutex.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>