gdrdrv returns the same handle for two pinned buffers

[pakmarkthub]

Under the following conditions, gdrdrv returns the same handle for two different pinned buffers:

1. The size of the first buffer is smaller than the GPU page.
2. The start of the second buffer is within the same page of the first buffer.

The handle for the second buffer supersedes that of the first buffer. One scenario that this behavior becomes problematic is:

• The first buffer is page-aligned.
• The second buffer is not page-aligned.

This will run correctly.

gdr_pin_buffer(g, first_buffer_ptr, first_buffer_size, 0, 0, &first_buffer_handle);
gdr_map(g, first_buffer_handle, &first_buffer_map_ptr, first_buffer_size);
gdr_pin_buffer(g, second_buffer_ptr, second_buffer_size, 0, 0, &second_buffer_handle);

This will fail because the offset of the second buffer is non-zero. But the failure happens when trying to map the first buffer!

gdr_pin_buffer(g, first_buffer_ptr, first_buffer_size, 0, 0, &first_buffer_handle);
gdr_pin_buffer(g, second_buffer_ptr, second_buffer_size, 0, 0, &second_buffer_handle);
gdr_map(g, first_buffer_handle, &first_buffer_map_ptr, first_buffer_size);

Code in #242 can be used to reproduce this bug.