bpf, docs: Fix final bpf docs build failure

bpf, docs: Fix final bpf docs build failure

maps.rst in the BPF documentation links to the
/userspace-api/ebpf/syscall document
(Documentation/userspace-api/ebpf/syscall.rst). For some reason, if you
try to reference the document with :doc:, the docs build emits the
following warning:

./Documentation/bpf/maps.rst:13: WARNING: \
    unknown document: '/userspace-api/ebpf/syscall'

It appears that other places in the docs tree also don't support using
:doc:. Elsewhere in the BPF documentation, we just reference the kernel
docs page directly. Let's do that here to clean up the last remaining
noise in the docs build.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230302183918.54190-2-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>