virt/sev-guest: Return -EIO if certificate buffer is not large enough

Commit

  47894e0f ("virt/sev-guest: Prevent IV reuse in the SNP guest driver")

changed the behavior associated with the return value when the caller
does not supply a large enough certificate buffer. Prior to the commit a
value of -EIO was returned. Now, 0 is returned.  This breaks the
established ABI with the user.

Change the code to detect the buffer size error and return -EIO.

Fixes: 47894e0f

 ("virt/sev-guest: Prevent IV reuse in the SNP guest driver")
Reported-by: Larry Dewey <larry.dewey@amd.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Larry Dewey <larry.dewey@amd.com>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/2afbcae6daf13f7ad5a4296692e0a0fe1bc1e4ee.1677083979.git.thomas.lendacky@amd.com