uml: fix build error due to frame size > 1024

uml: fix build error due to frame size > 1024

the UML build fails during the kernel build:
| arch/um/drivers/net_kern.c: In function 'compute_hash':
| arch/um/drivers/net_kern.c:322:1: error: the frame size of 1072 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
|  322 | }
|      | ^
|cc1: all warnings being treated as errors

The compute_hash() function is added by our patch:
102-pseudo-random-mac.patch

Instead of allocating a 1024 byte buffer on the stack for the SHA1
digest input, let's allocate the data on the heap. We should be
able to do that since crypto_alloc_ahash and ahash_request_alloc
also need to allocate structures on the heap.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>