exfat: add balloc parameter to exfat_map_cluster() for iomap support

exfat: add balloc parameter to exfat_map_cluster() for iomap support

In preparation for supporting the iomap infrastructure, we need to know
whether a new cluster was allocated or not in exfat_map_cluster().

Add an optional 'bool *balloc' output parameter. When a new cluster is
allocated, *balloc is set to true. Pass NULL from exfat_get_block() to
preserve the existing behavior.

Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>