vvfat: React to bdrv_is_allocated() errors

vvfat: React to bdrv_is_allocated() errors

If bdrv_is_allocated() fails, we should react to that failure.
For 2 of the 3 callers, reporting the error was easy.  But in
cluster_was_modified() and its lone caller
get_cluster_count_for_direntry(), it's rather invasive to update
the logic to pass the error back; so there, I went with merely
documenting the issue by changing the return type to bool (in
all likelihood, treating the cluster as modified will then
trigger a read which will also fail, and eventually get to an
error - but given the appalling number of abort() calls in this
code, I'm not making it any worse).

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>