keys: fix sparse warning by adding __user annotation to cast

Fix the following sparse warning:

      CC      security/keys/key.o
    security/keys/keyctl.c:1297:10: warning: incorrect type in argument 2 (different address spaces)
    security/keys/keyctl.c:1297:10:    expected char [noderef] <asn:1>*buffer
    security/keys/keyctl.c:1297:10:    got char *<noident>

which appears to be caused by lack of __user annotation to the cast of
a syscall argument.

Signed-off-by: James Morris <jmorris@namei.org>
Acked-by: David Howells <dhowells@redhat.com>