JFFS2 update; completed support for NAND flash.

JFFS2 update; completed support for NAND flash.

 - Implement write-buffer flushing by garbage collection instead of padding.
 - Implement selective write-buffer flushing on fsync().
 - Implement error recovery on write-buffer flush.
 - Fix remove_suid(). Writing to a suid file didn't previously mark the file non-suid.
 - Fix handling of full file systems, to avoid unlink() returning -ENOSPC.
 - Fix assorted memory leaks.
 - Improve garbage collection efficiency by merging fewer pages.