bpf, docs: Fix verifier references

Use normal RST file reference instead of linkage copied from the old filter.rst
document that does not actually work when using HTML output.

Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/instruction-set.rst

···
     bpf_exit
 
   After the call the registers R1-R5 contain junk values and cannot be read.
-  An in-kernel `eBPF verifier`_ is used to validate eBPF programs.
+  An in-kernel verifier.rst is used to validate eBPF programs.
 
 Also in the new design, eBPF is limited to 4096 insns, which means that any
 program will terminate quickly and will only call a fixed number of kernel
···
 filters may use it as assembler to generate code from kernel. In kernel usage
 may not be bounded by security considerations, since generated eBPF code
 may be optimizing internal code path and not being exposed to the user space.
-Safety of eBPF can come from the `eBPF verifier`_. In such use cases as
+Safety of eBPF can come from the verifier.rst. In such use cases as
 described, it may be used as safe instruction set.
 
 Just like the original BPF, eBPF runs within a controlled environment,
···
 instruction that loads 64-bit immediate value into a dst_reg.
 Classic BPF has similar instruction: ``BPF_LD | BPF_W | BPF_IMM`` which loads
 32-bit immediate value into a register.
-
-.. Links:
-.. _eBPF verifier: verifiers.rst