bonding: move slave MTU handling from sysfs V2

V1->V2: corrected res/ret use

For some reason, MTU handling (storing, and restoring) is taking  place in
bond_sysfs. The correct place for this code is in bond_enslave, bond_release.
So move it there.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>