The maximum size of JFFS2 is 4GB minus the block size of the flash.
Enforce this limitation when setting up a filesystem on a flashdev
region that is larger than the maximum.
This also resolves some off-nominal memory leaks.
This adds generic JFFS2 interworking code that allows JFFS2 to be used
on top of any flashdev backend. It currently only supports NOR flashdev
backends.
