Use locking for bitmaps to avoid race conditions

[LennartF22]

Prevent concurrent access to the ifree and bfree bitmaps by adding a file system wide mutex for each bitmap. Before, race conditions, which potentially corrupt the whole file system, could occur.

[pjungkamp]

The critical sections should be protected using sb_start_write and sb_end_write and their siblings. That should suffice to protect the bitmaps. But ouichefs would need way more than that to be correct on a SMP system anyway.