makefs: warn that ffs sectorsize other than 512 may not work

newfs always sets sectorsize to DEV_BSIZE (512) and derives some other
values based on the number of 512-byte sectors per real sector. Similar
logic is required in makefs. Until that happens, emit a warning that
the image may be incorrect.

PR: 276571
(cherry picked from commit 176b9e0d724b4c1a24d2c46a1256381784f04edc)