bpf: add missing percpu_counter_destroy() in htab_map_alloc()

syzbot is reporting ODEBUG bug in htab_map_alloc() [1], for
commit 86fe28f7 ("bpf: Optimize element count in non-preallocated
hash map.") added percpu_counter_init() to htab_map_alloc() but forgot to
add percpu_counter_destroy() to the error path.

Link: https://syzkaller.appspot.com/bug?extid=5d1da78b375c3b5e6c2b

 [1]
Reported-by: syzbot <syzbot+5d1da78b375c3b5e6c2b@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 86fe28f7 ("bpf: Optimize element count in non-preallocated hash map.")
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/e2e4cc0e-9d36-4ca1-9bfa-ce23e6f8310b@I-love.SAKURA.ne.jp


Signed-off-by: Alexei Starovoitov <ast@kernel.org>