UBI: do not allocate the memory unnecessarily

UBI reserves an LEB sized buffer for various needs. We can use this buffer
while scanning, instead of allocating another one. This patch was originally
created by Jan Luebbe <[e-mail]>, but then he dropped it and I picked
up and tweaked a little bit.

Signed-off-by: Artem Bityutskiy <[e-mail]>

drivers/mtd/ubi/attach.c

···
 int ubi_compare_lebs(struct ubi_device *ubi, const struct ubi_ainf_peb *aeb,
 			int pnum, const struct ubi_vid_hdr *vid_hdr)
 {
-	void *buf;
 	int len, err, second_is_newer, bitflips = 0, corrupted = 0;
 	uint32_t data_crc, crc;
 	struct ubi_vid_hdr *vh = NULL;
···
 	/* Read the data of the copy and check the CRC */
 
 	len = be32_to_cpu(vid_hdr->data_size);
-	buf = vmalloc(len);
-	if (!buf) {
-		err = -ENOMEM;
-		goto out_free_vidh;
-	}
 
-	err = ubi_io_read_data(ubi, buf, pnum, 0, len);
+	mutex_lock(&ubi->buf_mutex);
+	err = ubi_io_read_data(ubi, ubi->peb_buf, pnum, 0, len);
 	if (err && err != UBI_IO_BITFLIPS && !mtd_is_eccerr(err))
-		goto out_free_buf;
+		goto out_unlock;
 
 	data_crc = be32_to_cpu(vid_hdr->data_crc);
-	crc = crc32(UBI_CRC32_INIT, buf, len);
+	crc = crc32(UBI_CRC32_INIT, ubi->peb_buf, len);
 	if (crc != data_crc) {
 		dbg_bld("PEB %d CRC error: calculated %#08x, must be %#08x",
 			pnum, crc, data_crc);
···
 		dbg_bld("PEB %d CRC is OK", pnum);
 		bitflips = !!err;
 	}
+	mutex_unlock(&ubi->buf_mutex);
 
-	vfree(buf);
 	ubi_free_vid_hdr(ubi, vh);
 
 	if (second_is_newer)
···
 
 	return second_is_newer | (bitflips << 1) | (corrupted << 2);
 
-out_free_buf:
-	vfree(buf);
+out_unlock:
+	mutex_unlock(&ubi->buf_mutex);
 out_free_vidh:
 	ubi_free_vid_hdr(ubi, vh);
 	return err;