KEYS: return full count in keyring_read() if buffer is too small

KEYS: return full count in keyring_read() if buffer is too small

Commit db3d5ed6bd92 ("KEYS: fix writing past end of user-supplied buffer
in keyring_read()") made keyring_read() stop corrupting userspace memory
when the user-supplied buffer is too small.  However it also made the
return value in that case be the short buffer size rather than the size
required, yet keyctl_read() is actually documented to return the size
required.  Therefore, switch it over to the documented behavior.

Note that for now we continue to have it fill the short buffer, since it
did that before (pre-v3.13) and dump_key_tree_aux() in keyutils arguably
relies on it.

Fixes: db3d5ed6bd92 ("KEYS: fix writing past end of user-supplied buffer in keyring_read()")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Cc: <stable@vger.kernel.org> # v3.13+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>