bpf: Increase MAX_NR_MAPS to 17 in test_verifier.c

map_fds[16] is the last one index-ed by fixup_map_array_small.
Hence, the MAX_NR_MAPS should be 17 instead.

Fixes: fb2abb73 ("bpf, selftest: test {rd, wr}only flags and direct value access")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>