MFC r204474: Fix the race between dotdot lookup and forced unmount, by using msdosfs-specific variant of vn_vget_ino(), msdosfs_deget_dotdot().
MFC r204474:
Fix the race between dotdot lookup and forced unmount, by using
msdosfs-specific variant of vn_vget_ino(), msdosfs_deget_dotdot().

As was done for UFS, relookup the dotdot denode after the call to
msdosfs_deget_dotdot(), because vnode lock is dropped and directory
might be moved.

MFC r204675:
When returning error from msdosfs_lookup(), make sure that *vpp is NULL.

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