Verify every block pointer is either embedded, hole, or has a valid DVA

Now instead of crashing when attempting to read the corrupt block
pointer, ZFS will return ECKSUM, in a stack that looks like this:

```
none:set-error
zfs.ko`arc_read+0x1d82
zfs.ko`dbuf_read+0xa8c
zfs.ko`dmu_buf_hold_array_by_dnode+0x292
zfs.ko`dmu_read_uio_dnode+0x47
zfs.ko`zfs_read+0x2d5
zfs.ko`zfs_freebsd_read+0x7b
kernel`VOP_READ_APV+0xd0
kernel`vn_read+0x20e
kernel`vn_io_fault_doio+0x45
kernel`vn_io_fault1+0x15e
kernel`vn_io_fault+0x150
kernel`dofileread+0x80
kernel`sys_read+0xb7
kernel`amd64_syscall+0x424
kernel`0xffffffff810633cb
```

This patch should hopefully also prevent such corrupt block pointers
from being written to disk in the first place.

And in zdb, don't crash when printing a block pointer with no valid
DVAs.  If a block pointer isn't embedded yet doesn't have any valid
DVAs, that's a data corruption bug.  zdb should be able to handle the
situation gracefully.

Finally, remove an extra check for gang blocks in SNPRINTF_BLKPTR.  This
check, which compares the asizes of two different DVAs within the same
BP, was added by illumos-gate commit b24ab67[^1], and I can't understand
why.  It doesn't appear to do anything useful, so remove it.

[^1]: illumos/illumos-gate@b24ab67

Fixes		openzfs#17077
Sponsored by:	ConnectWise
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed by: Alek Pinchuk <[email protected]>
Signed-off-by:	Alan Somers <[email protected]>
Closes openzfs#17078

Author: asomers

include/sys/spa.h

@@ -670,7 +670,6 @@ typedef struct blkptr {
 			    (u_longlong_t)DVA_GET_ASIZE(dva),		\
 			    ws);					\
 		}							\
-		ASSERT3S(copies, >, 0);					\
 		if (BP_IS_ENCRYPTED(bp)) {				\
 			len += func(buf + len, size - len,		\
 			    "salt=%llx iv=%llx:%llx%c",			\
@@ -679,10 +678,6 @@ typedef struct blkptr {
 			    (u_longlong_t)BP_GET_IV2(bp),		\
 			    ws);					\
 		}							\
-		if (BP_IS_GANG(bp) &&					\
-		    DVA_GET_ASIZE(&bp->blk_dva[2]) <=			\
-		    DVA_GET_ASIZE(&bp->blk_dva[1]) / 2)			\
-			copies--;					\
 		len += func(buf + len, size - len,			\
 		    "[L%llu %s] %s %s %s %s %s %s %s%c"			\
 		    "size=%llxL/%llxP birth=%lluL/%lluP fill=%llu%c"	\

module/zfs/zio.c

@@ -1212,6 +1212,16 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
 			    bp, (longlong_t)BPE_GET_PSIZE(bp));
 		}
 		return (errors ? ECKSUM : 0);
+	} else if (BP_IS_HOLE(bp)) {
+		/*
+		 * Holes are allowed (expected, even) to have no DVAs, no
+		 * checksum, and no psize.
+		 */
+		return (errors ? ECKSUM : 0);
+	} else if (unlikely(!DVA_IS_VALID(&bp->blk_dva[0]))) {
+		/* Non-hole, non-embedded BPs _must_ have at least one DVA */
+		errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
+		    "blkptr at %px has no valid DVAs", bp);
 	}
 	if (unlikely(BP_GET_CHECKSUM(bp) >= ZIO_CHECKSUM_FUNCTIONS)) {
 		errors += zfs_blkptr_verify_log(spa, bp, blk_verify,