linux sendfile: Fix handling of non-blocking sockets

FreeBSD sendfile() may perform a partial transfer and return EAGAIN if
the socket is non-blocking. Linux sendfile() expects no error in this
case, so squash EAGAIN.

PR: 282495
Tested by: pieter@krikkit.xyz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47447