The code works like follow (N = bit_len):

	N	-1	%4	2<<	-1	~ (binary)
	--------------------------------------------------
	1	0	0	2	1	1111 1110
	2	1	1	4	3	1111 1100
	3	2	2	8	7	1111 1000
	4	3	3	16	15	1111 0000
	5	4	0	2	1	1111 1110
	6	5	1	4	3	1111 1100
	7	6	2	8	7	1111 1000
	8	7	3	16	15	1111 0000
	...	...	...	...	...	...

Addresses a bug reported by FangfangLi <ffli@syntest.com.cn>.

[dbrownell@users.sourceforge.net: fix spelling bug too]

Signed-off-by: Michael Roth <mroth@nessie.de>
Cc: FangfangLi <ffli@syntest.com.cn>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>