NFSD: Implement FATTR4_CLONE_BLKSIZE attribute

NFSD: Implement FATTR4_CLONE_BLKSIZE attribute

RFC 7862 states that if an NFS server implements a CLONE operation,
it MUST also implement FATTR4_CLONE_BLKSIZE. NFSD implements CLONE,
but does not implement FATTR4_CLONE_BLKSIZE.

Note that in Section 12.2, RFC 7862 claims that
FATTR4_CLONE_BLKSIZE is RECOMMENDED, not REQUIRED. Likely this is
because a minor version is not permitted to add a REQUIRED
attribute. Confusing.

We assume this attribute reports a block size as a count of bytes,
as RFC 7862 does not specify a unit.

Reported-by: Roland Mainz <roland.mainz@nrubsig.org>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
Cc: stable@vger.kernel.org # v6.7+
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>