osd: add new 'copy-from2' operation

osd: add new 'copy-from2' operation

The new 'copy-from2' is very similar to the original 'copy-from'
operation.  The differences are:

 - It validates the flags and return an error if there are invalid bits
   set in it;
 - It processes a new 'copy-from' flag (*_TRUNCATE_SEQ) which allows a
   client to send 2 extra parameters, truncate_seq and truncate_size.
   These will be used for the destination object instead of simply being
   copied from the source object.

Signed-off-by: Luis Henriques <lhenriques@suse.com>