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.

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