nfs: create a kernel keyring

Create a kernel .nfs keyring similar to the nvme .nvme one.  Unlike for
a userspace-created keyrind, tlshd is a possesor of the keys with this
and thus the keys don't need user read permissions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://lore.kernel.org/r/20250515115107.33052-3-hch@lst.de


Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>