[JFFS2] Improve failure mode if inode checking leaves unchecked space.

We should never find the unchecked size is non-zero after we've finished
checking all inodes. If it happens, used to BUG(), leaving the alloc_sem
held and deadlocking. Instead, just return -ENOSPC after complaining. The
GC thread will die, but read-only operation should be able to continue and
the file system should be unmountable.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>