efi: stmm: Fix incorrect buffer allocation method

In the Linux kernel, the following vulnerability has been resolved:

efi: stmm: Fix incorrect buffer allocation method

The communication buffer allocated by setup_mm_hdr() is later on passed to tee_shm_register_kernel_buf(). The latter expects those buffers to be contiguous pages, but setup_mm_hdr() just uses kmalloc(). That can cause various corruptions or BUGs, specifically since commit 9aec2fb0fd5e ("slab: allocate frozen pages"), though it was broken before as well.

Fix this by using alloc_pages_exact() instead of kmalloc().