vfs_default: properly track written bytes for copy-chunk

vfs_default: properly track written bytes for copy-chunk

No change in behavour, this just makes the logic slightly more
understandable. In theory it would also allow the logic to be adjusted for
allowing short reads which is not quite clear from MS-SMB2 if we should allow
it. The file could be truncated while we're reading it.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12033

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
(cherry picked from commit e2d524d4baee193b0d03df3e7edda48b3cb4bddf)