dma-buf: heaps: Fix potential spectre v1 gadget

It appears like nr could be a Spectre v1 gadget as it's supplied by a
user and used as an array index. Prevent the contents
of kernel memory from being leaked to userspace via speculative
execution by using array_index_nospec.

Signed-off-by: Jordy Zomer <jordy@pwning.systems>
Fixes: c02a81fb ("dma-buf: Add dma-buf heaps framework")
Cc: <stable@vger.kernel.org> # v5.6+
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
 [sumits: added fixes and cc: stable tags]
Link: https://patchwork.freedesktop.org/patch/msgid/20220129150604.3461652-1-jordy@pwning.systems