dma_get_sgtable() tries to create a scatterlist table containing valid
struct page pointers for the coherent memory allocation passed in to it.

However, memory can be declared via dma_declare_coherent_memory(), or
via other reservation schemes which means that coherent memory is not
guaranteed to be backed by struct pages.  In such cases, the resulting
scatterlist table contains pointers to invalid pages, which causes
kernel oops later.

This patch adds detection of such memory, and refuses to create a
scatterlist table for such memory.
Reported-by: NShuah Khan <shuahkhan@gmail.com>
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>