isofs: Fix isofs_show_options()

The isofs patch needs a small fix to handle a signed/unsigned comparison that
the compiler didn't flag - thanks to Dan for catching it.

It should be noted, however, the session number handing appears to be incorrect
between where it is parsed and where it is used.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>