newfs(8): Fix a bug in initialization of sblock.fs_maxbsize .

Fixes: 1c85e6a35d93195e896b030d9a55f7ac4ccee2c3 (SVN r98542)
Pull Request: https://github.com/freebsd/freebsd-src/pull/587
Sponsored by: NVIDIA Networking
Approved by: re (gjb)

(cherry picked from commit c5f549c1e0c9aa1764e87d55f183053c1545b1fc)
(cherry picked from commit 4391c305e83552ed7801525723ff6c10a4efdf5e)