exfat: support multi-cluster for exfat_map_cluster

exfat: support multi-cluster for exfat_map_cluster

This patch introduces a parameter 'count' to support fetching multiple
clusters in exfat_map_cluster. The returned 'count' indicates the number
of consecutive clusters, or 0 when the input cluster offset is past EOF.

And the 'count' is also an input parameter for the caller to specify the
required number of clusters.

Only NO_FAT_CHAIN files enable multi-cluster fetching in this patch.

After this patch, the time proportion of exfat_get_block has decreased,
The performance data is as follows:

Cluster size: 512 bytes
Sequential read of a 30GB NO_FAT_CHAIN file:
2.4GB/s -> 2.5 GB/s
proportion of exfat_get_block:
10.8% -> 0.02%

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