docs: Fix dead references to kernel documentation

docs: Fix dead references to kernel documentation

Most date back to reorganisation of the Documentation/ tree in 2016,
new paths based on v5.7-rc5:

fdisk.8:   Documentation/{,admin-guide/}devices.txt
eject.c:   Documentation/{,userspace-api/}ioctl/cdrom.rst
mount.8:   Documentation/filesystems/adfs.{txt,rst}
rfkill.8:  Documentation/{rfkill.txt,driver-api/rfkill.rst}
tunelp.8:  Documentation/{parport.txt,admin-guide/parport.rst}
zramctl.8: Documentation/{blockdev/zram.txt,admin-guide/blockdev/zram.rst}

Signed-off-by: Yannick Le Pennec <yannick.lepennec@live.fr>