MFC r268855: MFV r268848:

MFC r268855: MFV r268848:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@269416 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f