openssh: Be more restrictive on private key file permissions

openssh: Be more restrictive on private key file permissions

Sometimes default permissions on filesystems can be more permissive
e.g. 0644, this can make the private key file created here to inherit
those permissions and these permissions can then cause ssh server to
not allow ssh connections due to non-secure permissions on file.

Reported-by: Jean-Michel Papy <jean-michel.papy@exail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Jörg Sommer <joerg.sommer@navimatix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>