This patch introduces sfds_protected, which allows
to mask areas, where modifications of sfds are prohibited.
That guarantees, that populated sfds won't be reused.

v4: New
v5: Add comment and print sfd type before BUG().
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>