exfat: return the start of next cache in exfat_cache_lookup

exfat: return the start of next cache in exfat_cache_lookup

Change exfat_cache_lookup to return the cluster number of the last
cluster before the next cache (i.e., the end of the current cache range)
or the given 'end' if there is no next cache. This allows the caller to
know whether the next cluster after the current cache is cached.

The function signature is changed to accept an 'end' parameter, which
is the upper bound of the search range. The function now stops early
if it finds a cache that starts within the current cache's tail, meaning
caches are contiguous. The return value is the cluster number at which
the next cache starts (minus one) or the original 'end' if no next cache
is found.

The new behavior is illustrated as follows:

cache:  [ccccccc-------ccccccccc]
search: [..................]
return:               ^

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>