bcachefs: fix smatch data leak warning in fs usage ioctl

smatch warns that the copy of arg to userspace is a potential data
leak by virtue of arg.pad not being checked or zeroed. This was
introduced by the commit referenced below that switched arg from
being a zeroed runtime allocation to living on the stack. Fix by
simply zero initializing the structure.

Fixes: cde738a61e65 ("bcachefs: Convert bch2_ioctl_fs_usage() to new accounting")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>