Linux 5.1 compat: get_ds() removed

Commit torvalds/linux@736706bee has removed the get_fs() function
as a bit of cleanup. It has been defined as KERNEL_DS on all
architectures for all supported kernels. Replace get_fs() with
KERNEL_DS as was done in the kernel.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8479