exfat: reduce unnecessary writes during mmap write

exfat: reduce unnecessary writes during mmap write

During mmap write, exfat_page_mkwrite() currently extends
valid_size to the end of the VMA range. For a large mapping,
this can push valid_size far beyond the page that actually
triggered the fault, resulting in unnecessary writes.

valid_size only needs to extend to the end of the page
being written.

Signed-off-by: Yuling Dong <yuling-dong@qq.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>