The function viafb_cursor() uses 2 stack-variables of CURSOR_SIZE bits;
CURSOR_SIZE is defined as (8 * 1024). Using up twice 1k on stack is too
much for 4k-stack (though it works with 8k-stacks). Make those two
variables kzalloc'ed to preserve stack space.
Also merge the whole lot of local struct's in viafb_ioctl into a union so
the stack usage gets minimized here as well. (struct's are only accessed
in their indicidual IOCTL case) This second part is only compile-tested as
I know of no userspace app using the IOCTLs.