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)