MFC of r269303:

When restoring a UFS dump onto a ZFS filesystem, an assertion in
restore was failing because ZFS was reporting a blocksize that was
not a multiple of 1024.  Replace restore's failed assertion with
code that writes restored files in a blocksize that works for
restore (a multiple of 1024) despite being non-optimal for ZFS.

Submitted by: Dmitry Morozovsky
Tested by:    Dmitry Morozovsky