Fix a logic bug in makefs lazy inode initialization.

We may need to initialize multiple inode blocks before writing a given
inode. makefs(8) was only initializing a single block at a time, so
certain inode allocation patterns could lead to a situation where it
wrote an inode to an uninitialized block. That inode might be clobbered
by a later initialization, resulting in a filesystem image containing
directory entries that point to a seemingly unused inode.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13505