passthrough_hp: excercise reusing inode numbers

passthrough_hp: excercise reusing inode numbers

Before last unlink() release the reference on inode.fd to allow reuse
of underlying fs inode number, mark the server inode "deleted" and bump
it's generation counter.

When same inode number is found on lookup(), the server inode object will
be reused as well.

Skip this when inode has an open file and when writeback cache is enabled.

This will be used to verify inode reuse bug fix in the kernel.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>