jffs2: Rename jffs2_erase_pending_trigger() to jffs2_dirty_trigger()

Now that we do erases from GC and trigger the GC thread to do them
instead of using kupdated, this function is misnamed. It's only used
for triggering wbuf flush on NAND flash now. Rename it accordingly.

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