selftests/bpf: Fix OOB write in test_verifier

The commit referenced below added fixup_map_timer support (to create a
BPF map containing timers), but failed to increase the size of the
map_fds array, leading to out of bounds write. Fix this by changing
MAX_NR_MAPS to 22.

Fixes: e60e6962 ("selftests/bpf: Add tests for restricted helpers")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211214014800.78762-1-memxor@gmail.com