MFV r271510:

Enforce 4K as smallest indirect block size (previously the smallest
indirect block size was 1K but that was never used).

This makes some space estimates more accurate and uses less memory
for some data structures.

Illumos issue:
    5141 zfs minimum indirect block size is 4K

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271526 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f