Squashfs: fix breakage when page size > metadata block size

Squashfs is broken on any system where the page size is larger than
the metadata size (8192).  This is easily fixed by ensuring cache->pages
is always > 0.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Doug Chapman <doug.chapman@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>