zdb: dump spill block pointer if present

zdb: dump spill block pointer if present

Output will look like so:

  $ sudo zdb -dddd -vv testpool/fs 2
  Dataset testpool/fs [ZPL], ID 260, cr_txg 8, 25K, 7 objects, rootbp DVA[0]=<0:1800be00:200> DVA[1]=<0:1c00be00:200> [L0 DMU objset] fletcher4 lz4 unencrypted LE contiguous unique double size=1000L/200P birth=16L/16P fill=7 cksum=d03b396cd:489ca835517:d4b04a4d0a62:1b413aac454d53

      Object  lvl   iblk   dblk  dsize  dnsize  lsize   %full  type
           2    1   128K    512     1K     512    512    0.00  ZFS plain file (K=inherit) (Z=inherit=lz4)
                                                 192   bonus  System attributes
      dnode flags: USED_BYTES USERUSED_ACCOUNTED USEROBJUSED_ACCOUNTED SPILL_BLKPTR
      dnode maxblkid: 0
      path    /testfile
      uid     0
      gid     0
      atime   Fri Jul 15 12:36:35 2022
      mtime   Fri Jul 15 12:36:35 2022
      ctime   Fri Jul 15 12:36:51 2022
      crtime  Fri Jul 15 12:36:35 2022
      gen 10
      mode    100600
      size    0
      parent  34
      links   1
      pflags  840800000004
      SA xattrs: 248 bytes, 2 entries

          security.selinux = nutanix_u:object_r:unlabeled_t:s0\000
          user.foo = xbLQJjyVvEVPGGuRHV/gjkFFO1MdehKnLjjd36ZaoMVaUqtqFoMMYT5Ya9yywHApJNoK/1hNJfO3\012XCJWv9/QUTKamoWW9xVDE7yi8zn166RNw5QUhf84cZ3JNLnw6oN

Spill block: 0:10005c00:200 0:14005c00:200 200L/200P F=1 B=16/16 cksum=1cdfac47a4:910c5caa557:195d0493dfe5a:332b6fde6ad547
Indirect blocks:

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Closes #13640