makefs(8): Fix calculation of file sizes

[kd]

Summary

When a new FS image is created we need to calculate how much space each file is going to consume.
Fix two bugs in that logic:

1. Count the space needed for indirection blocks for large files.
2. Normally the trailing data of a file is written to a block of frag size, 4 kB by default.

However for files that use indirect blocks a full block is allocated, 32kB by default.
Take that into account.

PR: 229929

[mckusick]

This is an accurate calculation the the extra needed space.

[asomers]

usr.sbin/makefs/ffs.c

628

One problem: the compiler complains about a signed/unsigned comparison in this line, as called from line 664. Could you please fix that?

[kd]

Cast both sides of comparison to size_t.

[kd]

Committed in ecdc04d006.