kernel: fixing a potential deadlock in block2mtd for kernel 3.6/3.8/3.9

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37842 3c298f89-4303-0410-b956-a3cf2f4a3e73