9pfs: refactor 'name_idx' -> 'nwalked' in v9fs_walk()

9pfs: refactor 'name_idx' -> 'nwalked' in v9fs_walk()

The local variable 'name_idx' is used in two loops in function v9fs_walk().
Let the first loop use its own variable 'nwalked' instead, which we will
use in subsequent patch as the number of (requested) path components
successfully walked by background I/O thread.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <d506308e7e343023c4db95d0e6053dd2627ed3c1.1647339025.git.qemu_oss@crudebyte.com>