supplicant: adjust fragment_size according to MTU (bgo #755145)

supplicant: adjust fragment_size according to MTU (bgo #755145)

NetworkManager set wpa_supplicant's fragment_size option to 1300. But if MTU
was lower, wpa_supplicant failed with "l2_packet_send - sendto: Message too
long" due to fragmentation of EAP-TLS or EAP-PEAP packets.

Actually, MTU has to be 14 bytes bigger than the "fragment_size" parameter.

Ideally, wpa_supplicant would take MTU in the account and adjust the
fragmentation limit accordingly. See discussion in
http://lists.shmoo.com/pipermail/hostap/2015-August/033546.html

https://bugzilla.gnome.org/show_bug.cgi?id=755145