Changes

dev_create: support returning -EEXIST

Check if netlink has set errno to EEXIST and return -EEXIST in
gtp_dev_create() and gtp_dev_create_sgsn(). Users of the function can
use this to delete the GTP device and try creating it again.

Related: SYS#7240
Change-Id: Ib99bd8eed854014a5c9118c23e4058a41f3145f2