ubi: provide a way to skip CRC checks

Some users of static UBI volumes implement their own integrity check,
thus making the volume CRC check done at open time useless. For
instance, this is the case when one use the ubiblock + dm-verity +
squashfs combination, where dm-verity already checks integrity of the
block device but this time at the block granularity instead of verifying
the whole volume.

Skipping this test drastically improves the boot-time.

Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/ubi.h

@@ -327,6 +327,9 @@ struct ubi_eba_leb_desc {
  *           atomic LEB change
  *
  * @eba_tbl: EBA table of this volume (LEB->PEB mapping)
+ * @skip_check: %1 if CRC check of this static volume should be skipped.
+ *		Directly reflects the presence of the
+ *		%UBI_VTBL_SKIP_CRC_CHECK_FLG flag in the vtbl entry
  * @checked: %1 if this static volume was checked
  * @corrupted: %1 if the volume is corrupted (static volumes only)
  * @upd_marker: %1 if the update marker is set for this volume
@@ -374,6 +377,7 @@ struct ubi_volume {
 	void *upd_buf;
 
 	struct ubi_eba_table *eba_tbl;
+	unsigned int skip_check:1;
 	unsigned int checked:1;
 	unsigned int corrupted:1;
 	unsigned int upd_marker:1;