linux-cp: fix tap interface attrs in case the sw pool realloc'd

linux-cp: fix tap interface attrs in case the sw pool realloc'd

Creating tap interface / sub interface causes allocation of a new
software interface with possible sw interface pool reallocation.
In such case accessing L3 MTU and interface flags by obsolete sw
pointer is UAF.
Instead, keep desired tap interface MTU value before sw intreface
creation and refetch sw pointer right before sw flags inheritance.

Type: fix
Fixes: b89c1ddcb3b4f9138ca3ebefb2115f896ff3e1bd
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: I21ea46d146d11060bb9bedc77377ab17ae9e22e8