[ng_socket] Don't take the SOCKBUF_LOCK() twice in the RX data path.

This is just a minor optimization, but it's sensitive. This gives an improvement of 30-50 kpps.

Reviewed by: kp, markj, glebius, lutz_donnerhacke.de
Approved by: vmaffione (mentor)
Sponsored by: vstack.com
Differential Revision: https://reviews.freebsd.org/D27382