Fix data corruption when cloning embedded blocks

Don't overwrite blk_phys_birth, as for embedded blocks it is part of
the payload.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Issue #13392
Closes #14739