Interface MTU has to be equal or less then `32767` (`2^15-1`), but it should be `2^16-1`

Environment

• Python version: 3.5.2
• NetBox version: 2.3.8-dev

Steps to Reproduce

Edit an interface of a device, and set an MTU of i.e. 65536.

Expected Behavior

Accept an 16-bit value, or at least 16-bit minus 1.

Observed Behavior

Netbox threw an error: "Ensure this value is less than or equal to 32767."

Additional notes

I'm somehow confused that Linux has an MTU for the loopback interface lo a value of 65536, seen by

# cat /sys/class/net/lo/mtu
65536
# uname -r
4.17.0-1-amd64

but regarding to https://tools.ietf.org/html/rfc791 the maximum value of MTU is just 65535 (2^16-1).

The value of 2^16 comes directly from https://github.com/torvalds/linux/blob/master/drivers/net/loopback.c#L176

	dev->mtu	= 64 * 1024;

So one thing is that the max. MTU in netbox is not as big as the RFC says it can be; and why David and Eric had chosen 2^16 instead of 2^16-1 is another :)

[jeremystretch]

This is because we use a PositiveSmallIntegerField to store the MTU, which uses a signed 16-bit small integer for the database column but only allows positive values.

To accommodate values larger than 32767 we would need to migrate this to a PositiveIntegerField and add a validator to limit the maximum to 65535 (or 65536?).