Instead of asserting all zio's be properly aligned, only assert
on the logical ones.
Cap uberblocks at 8k, otherwise with ashift=17, there would be
only one uberblock.
This fixes a problem that zdb would trip assert on pools with
ashift >= 0xe (8k).
While there, also change the code so it only attempt to condense
space map unless the uncondensed size consumes greater than
zfs_metaslab_condense_block_threshold blocks.
Illumos issue:
4958 zdb trips assert on pools with ashift >= 0xe
MFC after: 2 weeks
7882b61f60