venus: fix format overflow warning

venus: fix format overflow warning

Recent versions of gcc generate a format overflow warning on the dfstring
buffer in fs.c.  Increase the size of the buffer to avoid a possible buffer
overflow.

    fs.c: In function ‘AclToString’:
    fs.c:770:30: error: ‘%s’ directive writing up to 1024 bytes
    into a region of size between 13 and 23 [-Werror=format-overflow=]
      sprintf(dfsstring, " dfs:%d %s", acl->dfs, acl->cell);
                                  ^~
    fs.c:770:2: note: ‘sprintf’ output between 8 and 1042 bytes into
    a destination of size 30
      sprintf(dfsstring, " dfs:%d %s", acl->dfs, acl->cell);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Iead8b153a62f2928fabaeee1ed126535f67d7d49
Reviewed-on: https://gerrit.openafs.org/12917
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>