bonding: convert num_grat_arp to the new bonding option API

num_grat_arp wasn't converted to the new bonding option API, so do this
now and remove the specific sysfs store option in order to use the
standard one. num_grat_arp is the same as num_unsol_na so add it as an
alias with the same option settings. An important difference is the option
name which is matched in bond_sysfs_store_option().

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>