We need to protect the read/write commands with a mutex.

Bug reported by Henrik Sjoberg <henke@epact.se>
Signed-off-by: NManu Abraham <manu@linuxtv.org>
Signed-off-by: NMichael Krufky <mkrufky@linuxtv.org>
Cc: Johannes Stezenbach <js@linuxtv.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>