linux-user: fix openat

When running openat using qemu-arm, we stumbled over invalid permissions
on the created files. The reason for this is that the mode parameter gets
treates as an O_... flag, which it isn't - it's a permission bitmask.

This patch removes the needless translation of the mode parameter,
rendering permission passing of openat() to work with linux-user.

Reported-by: Dirk Mueller <dmueller@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
