9pfs: fix inode sequencing in 'synth' driver

9pfs: fix inode sequencing in 'synth' driver

The 'synth' driver's root node and the 'synth' driver's first
subdirectory node falsely share the same inode number (zero), which
makes it impossible for 9p clients (i.e. 9p test cases) to distinguish
root node and first subdirectory from each other by comparing their QIDs
(which are derived by 9p server from driver's inode numbers).

Fix this issue by using prefix-increment instead of postfix-increment
operator while generating new inode numbers for subdirectories and files.

Link: https://lore.kernel.org/qemu-devel/3859307.hTDP4D0zbi@silver/
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <E1nTpyU-0000yR-9o@lizzy.crudebyte.com>