virt: sev-guest: Satisfy linear mapping requirement in get_derived_key()

Commit

  7ffeb2fc ("x86/sev: Document requirement for linear mapping of guest request buffers")

added a check that requires the guest request buffers to be in the linear
mapping. The get_derived_key() function was passing a buffer that was
allocated on the stack, resulting in the call to snp_send_guest_request()
returning an error.

Update the get_derived_key() function to use an allocated buffer instead
of a stack buffer.

Fixes: 7ffeb2fc ("x86/sev: Document requirement for linear mapping of guest request buffers")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/9b764ca9fc79199a091aac684c4926e2080ca7a8.1752698495.git.thomas.lendacky@amd.com