iplink_bond: fix parameter value matching

iplink_bond: fix parameter value matching

Lookup function get_index() compares argument with table entries
only up to the length of the table entry so that if an entry
with lower index is a substring of a later one, earlier entry is
used even if the argument is equal to the other. For example,

  ip link set bond0 type bond xmit_hash_policy layer2+3

sets xmit_hash_policy to 0 (layer2) as this is found before
"layer2+3" can be checked.

Use strcmp() to compare whole strings instead.

v2: look for an exact match only

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>