ext4: switch from spin_lock to mutex_lock

A spinlock is probably not the right thing to use
on our singlecore processor. This has a nice sideeffect;
The scheduler won't get confused if a process
tries to sleep in this critical section.

This should fix every kernel crash while trimming and
using the device.

Change-Id: I96bb3462875143d963eacb301aca89194340aaaa
Signe-off-by: Blechd0se <alex.christ@hotmail.de>