sunrpc: kernel BUG at kernel/cred.c:825!

Init missing debug member magic with CRED_MAGIC.

Signed-off-by: Santosh kumar pradhan <santoshkumar.pradhan@wdc.com>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>