simple_fill_super(): don't bother with d_genocide() on failure

Failing ->fill_super() will be followed by ->kill_sb(), which should
include kill_litter_super() if the call of simple_fill_super() had
been asked to create anything besides the root dentry.  So there's
no need to empty the partially populated tree - it will be trimmed
by inevitable kill_litter_super().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>