shmem: put_super must percpu_counter_destroy

list_add() corruption messages reported from shmem_fill_super()'s recently
introduced percpu_counter_init(): shmem_put_super() needs to remember to
percpu_counter_destroy().  And also check error from percpu_counter_init().

Reported-bisected-and-tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>