gofer: some fixs in setupRootFS
gofer: some fixs in setupRootFS

1.use root instead of spec.Root.path as mountpoint
2.put remount readonly logic ahead to avoid device busy errors

Signed-off-by: Liu Hua <ce25754916c22a624d2187bef6bc5c7c4d2bc887@huawei.com>
Change-Id: I9222b4695f917136a97b0898ac6f75fcff296e5d
PiperOrigin-RevId: 240818182

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor