mnt: Devpts options get corrupted on dump (v2)

The memcpy() in devpts_dump() just overwrites part of them.
Fix this and move the whole code into sub-routine for future.

v2: Fix off-by-one error spotted by Filipe.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Filipe Brandenburger <filbranden@google.com>