bpf: Fix error return code in map_lookup_and_delete_elem()

Fix to return negative error code -EFAULT from the copy_to_user() error
handling case instead of 0, as done elsewhere in this function.

Fixes: bd513cd0

 ("bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200430081851.166996-1-weiyongjun1@huawei.com