f2fs: allocate memory in batch in build_sit_info()

build_sit_info() allocate all bitmaps for each segment one by one,
it's quite low efficiency, this pach changes to allocate large
continuous memory at a time, and divide it and assign for each bitmaps
of segment. For large size image, it can expect improving its mount
speed.

Signed-off-by: Chen Gong <gongchen4@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.h

@@ -226,6 +226,7 @@ struct sit_info {
 	block_t sit_base_addr;		/* start block address of SIT area */
 	block_t sit_blocks;		/* # of blocks used by SIT area */
 	block_t written_valid_blocks;	/* # of valid blocks in main area */
+	char *bitmap;			/* all bitmaps pointer */
 	char *sit_bitmap;		/* SIT bitmap pointer */
 #ifdef CONFIG_F2FS_CHECK_FS
 	char *sit_bitmap_mir;		/* SIT bitmap mirror */