socket: Split up soreceive_generic()

Factor out the bits that run with the sock I/O lock held into a separate
function. No functional change intended.

Reviewed by: gallatin, glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D46304

(cherry picked from commit 0a68f644dca19670686007071479f919a56ea37f)