guard the setting and checking of i_usecount by simplelock; as
situation where there would be several users of same ntnode is
extremery rare, it's not worth the efford to change the code to
hold the (exclusive) lock only when it's needed
* in struct fnode, g/c f_dev & f_type (former is already in ntnode,
latter is in parent vnode), move f_devvp from struct fnode into
struct ntnode (no reason this should be copied in each fnode)
* f_lock is only needed for FreeBSD (for their totally disgusting
and insane way of doing generic vnode locking)
* g/c f_mp, it was write-only
* some comments added
is mounted and all ntfs volumes share it; the space occupied by the
table is freed upon last ntfs volumen umount. Saves 128KB for every
ntfs volume mounted.
XXX for now, read just first 256 entries - no more would be currently used
anyway