f2fs: avoid variable length array

Instead of using variable length array, this patch let preallocate memory for
them.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
index 421d579..ba858ca 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -189,6 +189,7 @@
 	char *sit_bitmap;		/* SIT bitmap pointer */
 	unsigned int bitmap_size;	/* SIT bitmap size */
 
+	unsigned long *tmp_map;			/* bitmap for temporal use */
 	unsigned long *dirty_sentries_bitmap;	/* bitmap for dirty sentries */
 	unsigned int dirty_sentries;		/* # of dirty sentries */
 	unsigned int sents_per_block;		/* # of SIT entries per block */