staging: gasket: page table: use GFP_KERNEL for dma_alloc_coherent

Flags should be specified for dma_alloc_coherent() call.  Use
GFP_KERNEL, it's fine to sleep here.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>