linux(4): Add sendfile fallback for non-socket fds

Before Linux 2.6.33, out_fd must refer to a socket. Since Linux 2.6.33
it can be any file.
The patch was originally provided by James McLaughlin and adapted by me
for copy_file_range.

PR: 262535
Differential revision: https://reviews.freebsd.org/D34555
MFC after: 1 month