include/linux/cred.h: work around gcc-4.2.4 warning in get_cred()

With gcc 4.2.4 (building UML) I get the warning

include/linux/cred.h: In function 'get_cred':
include/linux/cred.h:189: warning: passing argument 1 of
'get_new_cred' discards qualifiers from pointer target type

Inserting an additional local variable appears to keep the compiler happy,
although it's not clear to me why this should be needed.

Signed-off-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>