/* * Mark a mount point as busy, and gain a new reference to it. Used to * prevent the file system from being unmounted during critical sections. * * vfs_busy can be called multiple times and by multiple threads * and must be accompanied by the same number of vfs_unbusy calls. * * => The caller must hold a pre-existing reference to the mount. * => Will fail if the file system is being unmounted, or is unmounted. */