Fix missed assertion update in physical rewrite patch

Physical rewrite patch changed the meaning of BP_GET_BIRTH(), but
I missed update one of its occurences, ending up asserting equal
logical birth times instead of equal physical birth times.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Fixes #17565
Closes #17631