fs/fat/fatent.c: add cond_resched() to fat_count_free_clusters()

On non-preempt kernels this loop can take a long time (more than 50 ticks)
processing through entries.

Link: http://lkml.kernel.org/r/20181010172623.57033-1-khazhy@google.com
Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/fat/fatent.c

@@ -682,6 +682,7 @@ int fat_count_free_clusters(struct super_block *sb)
 			if (ops->ent_get(&fatent) == FAT_ENT_FREE)
 				free++;
 		} while (fat_ent_next(sbi, &fatent));
+		cond_resched();
 	}
 	sbi->free_clusters = free;
 	sbi->free_clus_valid = 1;