nfscl: fix uninitialized memory in nfsv4_loadattr

When processing an RPC response that did not include any Owner
attribute, nfsv4_loadattr would return na_uid and na_gid uninitialized.
The uninitialized values could then make their way into the NFS
attribute cache via nfscl_loadattrcache.

PR: 281279
Reported by: KMSAN
MFC after: 2 weeks
Reviewed by: rmacklem
Sponsored by: Axcient