Unionfs operations such as statfs() sometimes require access to
mount objects for the upper/lower filesystem layers. While the
upper/lower mount point vnodes will not be freed because the unionfs
mount object holds references to them, their v_mount fields will be
cleared if either underlying FS is forcibly unmounted. This will
result in a panic on any subsequent attempt to access the underlying
mount through a unionfs operation.
Instead, use vfs_ref_from_vp() to access the underlying mounts,
failing gracefully if vfs_ref_from_vp() returns NULL.
Found by running the unionfs stress2 test and then attempting to
execute 'df' after terminating the test.