ocfs2: Make ocfs2_slot_info private.

Just use osb_lock around the ocfs2_slot_info data.  This allows us to
take the ocfs2_slot_info structure private in slot_info.c.  All access
is now via accessors.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>