socket: Simplify synchronization in soreceive_stream()

Now that the socket recv I/O lock is required in order to enable
receive-side KTLS, we can move a check from out of the socket buffer
lock, which in turn will simplify some further refactoring.

Add a __predict_false annotation while here since we already perform a
lockless check before acquiring any locks.

Reviewed by: gallatin, jhb
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45922