keys: Protect the read method check against type removal

In the Linux kernel, the following vulnerability has been resolved:

keys: Protect the read method check against type removal

keyctl_read_key() checks key->type->read without holding key->sem.
A reference keeps the key alive, but does not pin the module containing
its type. A surviving RxRPC key can therefore be read while AF_RXRPC
is being unloaded:

KEYCTL_READ task Module unload and key GC
---------------- ------------------------
lookup_user_key()
permission / possession checks
load key->type
af_rxrpc_exit()
unregister_key_type()
GC: down_write(key->sem)
GC: retype to key_type_dead
GC: up_write(key->sem)
unregister_key_type() returns
free_module()
read saved type's read member

The final access can read freed module storage. The read invocation in
__keyctl_read_key() is already protected by key->sem, but the earlier
method check is outside that critical section.

Move the method check into __keyctl_read_key(), after key_validate() and
under its existing read hold on key->sem. The check and invocation then
use the same protected type for length queries and every read retry.
The GC cannot retype the key until the semaphore is released, and type
unregistration waits for that GC pass before the module can be freed.

Return -EOPNOTSUPP when validation succeeds and the type has no read
method. A temporary buffer may now be allocated before this result;
allocation and copying to userspace remain outside the semaphore.