bpf: Change kvfree to kfree in generic_map_lookup_batch()

bpf: Change kvfree to kfree in generic_map_lookup_batch()

buf_prevkey in generic_map_lookup_batch() is allocated with
kmalloc(). It's safe to free it with kfree().

Fixes: 25d6c96219f2 ("bpf: Add generic support for lookup batch op")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200601162814.17426-1-efremov@linux.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>