security: fix the key_permission LSM hook function type

Commit 8c0637e9 ("keys: Make the KEY_NEED_* perms an enum rather than
a mask") changed the type of the key_permission callback functions, but
didn't change the type of the hook, which trips indirect call checking with
Control-Flow Integrity (CFI). This change fixes the issue by changing the
hook type to match the functions.

Fixes: 8c0637e9 ("keys: Make the KEY_NEED_* perms an enum rather than a mask")
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <jmorris@namei.org>