Update NFS to handle larger link counts post ino64.

• Define a NFS_LINK_MAX as UINT32_MAX to match the wire protocol.
• Use NFS_LINK_MAX instead of LINK_MAX as the fallback value reported for a PATHCONF RPC by the NFS server.
• Use NFS_LINK_MAX instead of LINK_MAX as the default value reported by the NFS client pathconf() if not overridden by the NFS server.
• When reading the link count out of an RPC reply, read the full 32 bits instead of the lower 16 bits.

Reviewed by: rmacklem (earlier version)
Sponsored by: Chelsio Communications