nydusd: fix is_mount failure

[liubogithub]

is_mount can only take a canonicalized path which is shown in
/proc/self/mounts.

Signed-off-by: Liu Bo [email protected]

[imeoer]

LGTM

[bergwolf]

smoke failed

error[E0308]: mismatched types
   --> src/bin/nydusd/fusedev.rs:405:36
    |
405 |     let session = FuseSession::new(mnt, "rafs", "", readonly)?;
    |                                    ^^^
    |                                    |
    |                                    expected `&std::path::Path`, found struct `std::path::PathBuf`
    |                                    help: consider borrowing here: `&mnt`