exfat: fix soft lockup in exfat_clear_bitmap

bitmap clear loop will take long time in __exfat_free_cluster()
if data size of file/dir enty is invalid.
If cluster bit in bitmap is already clear, stop clearing bitmap go to
out of loop.

Fixes: 31023864 ("exfat: add fat entry operations")
Reported-by: Kun Hu &lt;huk23@m.fudan.edu.cn&gt;, Jiaji Qin <jjtan24@m.fudan.edu.cn>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>