nfscl: Make vfs.nfs.maxcopyrange larger by default

As of commit 103b207536f9, the NFSv4.2 server will limit the size
of a Copy operation based upon a 1 second timeout. The Linux 5.2
kernel server also limits Copy operation size to 4Mbytes.
As such, the NFSv4.2 client can attempt a large Copy without
resulting in a long RPC RTT for these servers.

This patch changes vfs.nfs.maxcopyrange to 64bits and sets
the default to the maximum possible size of SSIZE_MAX, since
a larger size makes the Copy operation more efficient and
allows for copying to complete with fewer RPCs.
The sysctl may be need to be made smaller for other non-FreeBSD
NFSv4.2 servers.

MFC after: 2 weeks