jffs2: Require jffs2_garbage_collect_trigger() to be called with lock held

We're about to call this from a bunch of places which already hold
c->erase_completion_lock, so add an assertion and change its existing
callers to do the same.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

diff --cc fs/jffs2/background.c
Simple merge

diff --cc fs/jffs2/nodemgmt.c
Simple merge

diff --cc fs/jffs2/super.c
Simple merge