mkfs.cramfs: lower memory requirements for layouts with duplicate files

mkfs.cramfs: lower memory requirements for layouts with duplicate files

mkfs.cramfs allocates memory based on a calculated upper-bound
of required filesystem size. If there are duplicate files
or hard links, the current implementation unnecessarily increases
the upper-bound per each copy of the file, even though cramfs does
not store copies of contents of identical files.

This patch improves the calculation of fslen_ub, the upper bound
of required filesystem size, by making the upper bound aware of
duplicate files.

This is very helpful for layouts that hold a lot of hard links,
which are seen as duplicate files by mkfs.cramfs. For example,
this drastically reduces the memory requirements for creating
a standard Busybox layout.

Signed-off-by: Roy Peled <the.roy.peled@gmail.com>