sev-dev: avoid pointless cred reference count bump

sev-dev: avoid pointless cred reference count bump

and fix a memory leak while at it. The new creds are created via
prepare_creds() and then reverted via put_cred(revert_creds()). The
additional reference count bump from override_creds() wasn't even taken
into account before.

Link: https://lore.kernel.org/r/20241125-work-cred-v2-8-68b9d38bb5b2@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>