NFS: Only reference user namespace from nfs4idmap struct instead of cred

The nfs4idmapper only needs access to the user namespace, and not the
entire cred struct. This replaces the struct cred* member with
struct user_namespace*. This is mostly hygiene, so we don't have to
hold onto the cred object, which has extraneous references to
things like user_struct. This also makes switching away
from init_user_ns more straightforward in the future.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>