makefs: Fix cd9660 filename buffer maximum length

The maximum length is 30 characters for name and extension, two
separators (. and ;) and 5 characters for file version from 1 to 32767,
which is 37 characters. Add one for the null term as we treat these
buffers as C strings.

This is not an issue in practice, as the file version is always 1 in
makefs.

While here, drop _WITH_PADDING from the macro name and update the
previously-unused ISO_FILENAME_MAXLENGTH for the corrected length.
A 0x00 padding byte is used by ISO9660 when needed for alignment, which
can be the null byte at the end of the string.

Use sizeof where appropriate.

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48193

(cherry picked from commit 2e09cef8dc6f46faba8bab87c42c3f19ba2ffe87)