Changes

dev_create: Avoid passing IFLA_GTP_FD* attribute with value -1

The kernel will just reject with errno=9 and log message:
"gtp: gtp socket fd=-1 not found".

Instead, avoid adding the attribute in first place, which is what the
kernel expects.

Related: SYS#7126
Change-Id: Ib7e1d362fbb556c9b15b25408058fbf00bcc7aa3