bonding: Allow userspace to set actors' system_priority in AD system

This patch allows user to randomize the system-priority in an ad-system.
The allowed range is 1 - 0xFFFF while default value is 0xFFFF. If user
does not specify this value, the system defaults to 0xFFFF, which is
what it was before this patch.

Following example code could set the value -
    # modprobe bonding mode=4
    # sys_prio=$(( 1 + RANDOM + RANDOM ))
    # echo $sys_prio > /sys/class/net/bond0/bonding/ad_actor_sys_prio
    # echo +eth1 > /sys/class/net/bond0/bonding/slaves
    ...
    # ip link set bond0 up

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
[jt: * fixed up style issues reported by checkpatch
     * changed how the default value is set in bond_check_params(), this
       makes the default consistent between what gets set for a new bond
       and what the default is claimed to be in the bonding options.]
Signed-off-by: Jonathan Toppins <jtoppins@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/bonding.txt

@@ -178,6 +178,15 @@ active_slave
 	active slave, or the empty string if there is no active slave or
 	the current mode does not use an active slave.
 
+ad_actor_sys_prio
+
+	In an AD system, this specifies the system priority. The allowed range
+	is 1 - 65535. If the value is not specified, it takes 65535 as the
+	default value.
+
+	This parameter has effect only in 802.3ad mode and is available through
+	SysFs interface.
+
 ad_select
 
 	Specifies the 802.3ad aggregation selection logic to use.  The