btrfs-progs: send: add gcc9 workaround for root item reset
    
    Gcc version 9.1.1 reports:
    
    In file included from /usr/include/string.h:494,
                     from kerncompat.h:25,
                     from ctree.h:26,
                     from send-utils.c:26:
    In function ‘memset’,
        inlined from ‘btrfs_read_root_item’ at send-utils.c:165:3,
        inlined from ‘subvol_uuid_search2’ at send-utils.c:494:8:
    /usr/include/bits/string_fortified.h:71:10: warning: ‘__builtin_memset’ offset [248, 439] from the object at ‘root_item’ is out of the bounds of referenced subobject ‘generation_v2’ with type ‘long long unsigned int’ at offset 239 [-Warray-bounds]
       71 |   return __builtin___memset_chk (__dest, __ch, __len, __bos0 (__dest));
          |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    That's correct in case the intent is to overwrite just the generation_v2
    member, but we want to zero the rest of the root item structure starting
    from the generation_v2. No typecasts can obscure that from gcc so the
    starting address is calculated as base pointer + member offset.
    
    Signed-off-by: David Sterba <dsterba@suse.com>