bpf: Fix memory leak on object 'data'

The error return path on when bpf_fentry_test* tests fail does not
kfree 'data'. Fix this by adding the missing kfree.

Addresses-Coverity: ("Resource leak")

Fixes: faeb2dce

 ("bpf: Add kernel test functions for fentry testing")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191118114059.37287-1-colin.king@canonical.com