[PATCH] get rid of corner case in dup3() entirely

Since Ulrich is OK with getting rid of dup3(fd, fd, flags) completely,
to hell the damn thing goes.  Corner case for dup2() is handled in
sys_dup2() (complete with -EBADF if dup2(fd, fd) is called with fd
that is not open), the rest is done in dup3().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>