gpu: ion: use vmalloc to allocate page array to map kernel

When ion_map_kernel is execute the system must allocate
an array large enough to hold a pointer to each page in
the buffer.  If the buffer is very large and the system
memory has become very fragmented, there may not be
sufficient high order allocations available from kmalloc.
Use vmalloc instead.

Change-Id: I912fc39d5e8fff3b210a0b2e22624b22e7965815
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: 11f463fe1ec5152c97c76e763be9f167ebb683df
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Laura Abbott <lauraa@codeaurora.org>