crypto: ccp - Fix the INIT_EX data file open failure

There are 2 common cases when INIT_EX data file might not be
opened successfully and fail the sev initialization:

1. In user namespaces, normal user tasks (e.g. VMM) can change their
   current->fs->root to point to arbitrary directories. While
   init_ex_path is provided as a module param related to root file
   system. Solution: use the root directory of init_task to avoid
   accessing the wrong file.

2. Normal user tasks (e.g. VMM) don't have the privilege to access
   the INIT_EX data file. Solution: open the file as root and
   restore permissions immediately.

Fixes: 3d725965 ("crypto: ccp - Add SEV_INIT_EX support")
Signed-off-by: Jacky Li <jackyli@google.com>
Reviewed-by: Peter Gonda <pgonda@google.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>