Btrfs-progs: fix the mismatch of extent buffer's space
    
    Now we set @refs to 2 on creating a new extent buffer, meanwhile we
    allocate the needed free space, but we don't give enough free_extent_buffer()
    to reduce the eb's references to zero so that the eb can finally be freed,
    so the problem is we has decrease the referene count of backrefs to zero, which
    ends up releasing the space occupied by the eb, and this space can be allocated
    again for something else(another eb or disk), usually a crash(core dump) will
    occur, I've hit a crash in rb_insert() because another eb re-use the space while
    the original one is floating around.
    
    We should do the same thing as the kernel code does, it's necessary to initialize
    @refs to 1 instead of 2, this helps us get rid of the above problem.
    
    Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
    Signed-off-by: David Sterba <dsterba@suse.cz>