ng_socket: Treat EEXIST from kern_kldload() as success

EEXIST is possible in a race condition.

Inspired by: ffc72591b1f5 (Don't worry if a module is already loaded ...)
Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44633

(cherry picked from commit f6f67f58c19db4f25f5c2cf4869efc7054493a55)
(cherry picked from commit 2e8d60c6854006a391491bc2c1c48c459c32e2c4)