staging: zsmalloc: Fix link error on ARM

staging: zsmalloc: Fix link error on ARM

commit d95abbbb291bf5bce078148f53603ce9c0aa1d44 upstream.

Testing the arm chromebook config against the upstream
kernel produces a linker error for the zsmalloc module from
staging. The symbol flush_tlb_kernel_range is not available
there. Fix this by removing the reimplementation of
unmap_kernel_range in the zsmalloc module and using the
function directly. The unmap_kernel_range function is not
usable by modules, so also disallow building the driver as a
module for now.

Signed-off-by: Joerg Roedel <joro@8bytes.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>