jffs2: Stop triggering block erases from jffs2_write_super()

This is the culmination of this sequence of patches. By moving the block
erasing from jffs2_write_super() into the GC code, we avoid huge
latencies on unmount where it waits for _all_ pending blocks to be
erased, and we allow better control for time-critical tasks by stopping
the GC thread.

Signed-off-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>