netfs: fix kernel doc for nets_wait_for_outstanding_io()

The @inode parameter wasn't documented leading to new doc build
warnings.

Fixes: f89ea63f

 ("netfs, 9p: Fix race between umount and async request completion")
Link: https://lore.kernel.org/r/20240528133050.7e09d78e@canb.auug.org.au
Signed-off-by: Christian Brauner <brauner@kernel.org>