vxlan: Use sk_release_kernel() instead of sock_release().

vxlan: Use sk_release_kernel() instead of sock_release().

A socket that is created using sock_create_kern() and then switched
to a namespace using sk_change_net() should be released using
sk_release_kernel() instead of sock_release().  This is because
sk_change_net() has already released a reference to the namespace
and we shouldn't try to release it again later.

Cc: Kyle Mestery <kmestery@cisco.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>