so_splice: Fix the KTLS check for the sink socket in so_splice()
Reviewed by: gallatin
Reported by: Claude Sonnet 4.6
Fixes: 1000cc4a0d39 ("so_splice: Disallow splicing with KTLS-enabled sockets")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56385