vfs_default: let copy_chunk_send use const from IDL

vfs_default: let copy_chunk_send use const from IDL

This also increases the buffer size from 8 MB to the current value of
COPYCHUNK_MAX_TOTAL_LEN which is 16 MB.

For the typical case when vfswrap_copy_chunk_send is called from the SMB
layer for an copy_chunk ioctl() the parameter "num" is guaranteed to be
at most 1 MB though.

It will only be larger for special callers like vfs_fruit for their
special implementation of copyfile where num will be the size of a file
to copy.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>