UBIFS: fix debugging failure in dbg_check_space_info

UBIFS: fix debugging failure in dbg_check_space_info

This patch fixes a debugging failure with which looks like this:
UBIFS error (pid 32313): dbg_check_space_info: free space changed from 6019344 to 6022654

The reason for this failure is described in the comment this patch adds
to the code. But in short - 'c->freeable_cnt' may be different before
and after re-mounting, and this is normal. So the debugging code should
make sure that free space calculations do not depend on 'c->freeable_cnt'.

A similar issue has been reported here:
http://lists.infradead.org/pipermail/linux-mtd/2011-April/034647.html

This patch should fix it.

For the -stable guys: this patch is only relevant for kernels 2.6.30
onwards.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: stable@kernel.org [2.6.30+]