makefs: Make cd9660 Rock Ridge inodes reproducible

Rock Ridge extensions include an inode field:

"POSIX File Serial Number" shall have the same meaning as and may be
used for the st_ino field of POSIX:5.6.1. This field shall be
recorded according to ISO 9660:7.3.3. Directory Records which share
the value of this field are defined as links (see POSIX:2.2.2.17)
and, by definition, point to the same file or directory.

Previously we'd store the source file's st_ino (except that in metalog
mode we'd record 0 for files with nlink = 1). This had two issues: the
generated ISO image was nonreproducible due to the arbitrary inode
numbers, and files without hard links would falsely be detected (by
certain tools) as hard links to each other.

Note that the kernel's cd9660(5) file system ignores the Rock Ridge
PX File Serial Number, so this issue isn't observed by mounting such a
file system.

Instead of using the source inode directly, assign target inode numbers
sequentially. Use a map so that files with the same source inode (hard
links) still receive the same target inode number.

PR: 284795
PR: 285027
Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49141