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>

Documentation/bpf/maps.rst

@@ -11,9 +11,9 @@ maps are accessed from BPF programs via BPF helpers which are documented in the
 `man-pages`_ for `bpf-helpers(7)`_.
 
 BPF maps are accessed from user space via the ``bpf`` syscall, which provides
-commands to create maps, lookup elements, update elements and delete
-elements. More details of the BPF syscall are available in
-:doc:`/userspace-api/ebpf/syscall` and in the `man-pages`_ for `bpf(2)`_.
+commands to create maps, lookup elements, update elements and delete elements.
+More details of the BPF syscall are available in `ebpf-syscall`_ and in the
+`man-pages`_ for `bpf(2)`_.
 
 Map Types
 =========
@@ -79,3 +79,4 @@ Find and delete element by key in a given map using ``attr->map_fd``,
 .. _man-pages: https://www.kernel.org/doc/man-pages/
 .. _bpf(2): https://man7.org/linux/man-pages/man2/bpf.2.html
 .. _bpf-helpers(7): https://man7.org/linux/man-pages/man7/bpf-helpers.7.html
+.. _ebpf-syscall: https://docs.kernel.org/userspace-api/ebpf/syscall.html