The in-memory slot map uses the same magic as the on-disk one.  There is
a special value to mark a slot as invalid.  It relies on the size of
certain types and so on.

Write a new in-memory map that keeps validity as a separate field.  Outside
of the I/O functions, OCFS2_INVALID_SLOT now means what it is supposed to.
It also is no longer tied to the type size.

This also means that only the I/O functions refer to 16bit quantities.
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>