PSA: Silent data corruption when receiving large blocks

commit 9ef375156a024e8b3b1eafaa5108f9db4a777097
Author: Fabian Keil <fk@fabiankeil.de>
Date:   Wed Sep 13 04:31:10 2017 +0200

    ZFS (XXX): Add vfs.zfs.risk_data_corruption sysctl
    
    ... and don't split large blocks while sending unless the
    sysctl is set.
    
    Splitting large blocks can lead to silent data corruption when
    receiving incremental streams with a block size change on the
    receiving side.
    
    Example corruption:
    
    [fk@test-vm ~]$ hd /usr/test/src/sbin/camcontrol/camcontrol.c
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    0003ea10
    [fk@test-vm ~]$ sudo zdb -ddddddd -bbbbbbb rpool/test/src $(stat -f '%i' /usr/test/src/sbin/camcontrol/camcontrol.c)
    Dataset rpool/test/src [ZPL], ID 550, cr_txg 67575, 704M, 78457 objects, rootbp DVA[0]=<0:254bfb000:1000> DVA[1]=<0:3803b7000:1000> [L0 DMU objset] sha256 uncompressed LE contiguous unique double size=800L/800P birth=67755L/67755P fill=78457 cksum=fb0fd08b71ffb3f8:e14c2e477498cdc4:4822440c7db2cccc:9fa4961bce0031b6
    
        Object  lvl   iblk   dblk  dsize  lsize   %full  type
        100844    2   128K   251K      0   251K    0.00  ZFS plain file (K=inherit) (Z=inherit)
                                            168   bonus  System attributes
            dnode flags: USED_BYTES USERUSED_ACCOUNTED
            dnode maxblkid: 0
            path    /sbin/camcontrol/camcontrol.c
            uid     1001
            gid     1001
            atime   Sat Jul  8 18:41:59 2017
[...]