socket: Microoptimize soreceive_stream_locked()

There is no need to hold the sockbuf lock while checking uio_resid.
No functional change intended.

MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield

(cherry picked from commit 6982be38cb7e4254ff4ffbb334cd7e234b5f6cc2)