Follow on to fix 1.51 for "Header with wrong dumpdate" message.

Must ensure that dump tapes from UFS1 filesystems properly copy
old fields of dump headers to new locations. Move check of dumpdate
to follow the code which ensures that the appropriate fields have
been copied.

PR: bin/118087
Help from: David Malone, Scott Lambert, Javier Martín Rueda
MFC after: 2 weeks