rtl8187: Fix crash on unload when using SLUB debug

Reported-by: Hin-Tak Leung <htl10@users.sourceforge.net>

After the code was modified to use urb anchors ("rtl8187: Use usb anchor
facilities to manage urbs"), rtl8187 began generating an intermittent GPF
on shutdown when using SLUB with debugging enabled. Furthermore, rebooting
the system with a ping running caused a GPF every time. There are two problems:
(1) incorrect locking in the rtl8187_rx_cb() routine, a pre-existing bug that
apparently had not been triggered before, and (2) duplicate freeing of receive
skbs that was probably introduced with the change to anchors.

Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>