Simplify copy-reflink code

[Ralph Boehme]

Hi David!

If you have the time, can you please take a look at

https://gitlab.com/samba-team/samba/-/merge_requests/3566 ?

I'm basically adding copy-reflink support to vfs_default alongside a new 
option "filesystem reflinks" that sets FILE_SUPPORTS_BLOCK_REFCOUNTING 
in the share capabilities if enabled..

This works on eg ZFS by just setting "filesystem reflinks = yes". 
vfs_btrfs still sets FILE_SUPPORTS_BLOCK_REFCOUNTING if the module is 
configured, so there's no behaviour change for btrfs. But for other 
filesystems we have the choice of either setting 
FILE_SUPPORTS_BLOCK_REFCOUNTING in the module or relying on the new option.

I had initially spotted a bug in vfs_btrfs and after fixing that [1], I 
realized that requiring all VFS modules to explicitly code copy-reflink 
support into them is not ideal and just moving it to vfs_default 
simplifies the modules a lot. The branch still contains the commit and 
its revert to demonstrate the problem.

tl;dr:

no change in behaviour, great simplification.

Thanks!
-slow


[1] 
<https://gitlab.com/samba-team/samba/-/merge_requests/3566/diffs?commit_id=48d8b9c7ad528790b24de5b1ccbfd8b04a2622cf>
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