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>