getmntpoint: Don't compare st_rdev for non-devices

If the mntfromname of a mountpoint is not a device (e.g. nullfs, tarfs,
procfs) we shouldn't compare st_rdev, as any match will be spurious.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51945

(cherry picked from commit 8073a5137f223bb481606b15edaa5ecb93ceffcb)