isofs: Fix off-by-one in 'session' mount option parsing

According to ECMA-130 standard maximum valid track number is 99. Since
'session' mount option starts indexing at 0 (and we add 1 to the passed
number), we should refuse value 99. Also the condition in
isofs_get_last_session() unnecessarily repeats the check - remove it.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>