->s_next_generation is protected by s_next_gen_lock but its usage
pattern is very primitive.  We don't actually need sequentially
increasing new generation numbers, so let's use prandom_u32() instead.
Reported-by: NDmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>