linux: fix handling of flags for 32 bit send(2) syscall

Previously the flags were passed as-is, which could resulted
in spurious EAGAIN returned for non-blocking sockets, which
broke some Steam games.

Approved by: re (gjb)
PR: 248065
Reported By: Alex S <iwtcex@gmail.com>
Tested By: Alex S <iwtcex@gmail.com>
Reviewed By: emaste
MFC After: 3 days
Sponsored By: The FreeBSD Foundation

(cherry picked from commit f6e8256a965d5b7a7d43034ea31b2430a3b51066)
(cherry picked from commit 9d930fb09072d21464288df8c834cb8d0b760073)