SUNRPC: create unix gid cache per network namespace

v2:
1) fixed silly usage of template cache as a real one (this code left from
static global cache for all)

This patch makes unix_gid_cache cache detail allocated and registered per
network namespace context.
Thus with this patch unix_gid_cache contents for network namespace "X" are
controlled from proc file system mount for the same network namespace "X".

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: J. Bruce Fields <bfields@redhat.com>