docs/bpf: Fix "file doesn't exist" warnings in {llvm_reloc,btf}.rst

scripts/documentation-file-ref-check reports warnings for (valid) cross-links
of form:

  :ref:`Documentation/bpf/btf <BTF_Ext_Section>`

Adding extension to the file name helps to avoid the warning, e.g:

  :ref:`Documentation/bpf/btf.rst <BTF_Ext_Section>`

Fixes: be4033d360 ("docs/bpf: Add description for CO-RE relocations")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Closes: https://lore.kernel.org/oe-kbuild-all/202309010804.G3MpXo59-lkp@intel.com
Link: https://lore.kernel.org/bpf/20230901125935.487972-1-eddyz87@gmail.com

Documentation/bpf/btf.rst

@@ -803,7 +803,7 @@ structure when .BTF.ext is generated. All ``bpf_core_relo`` structures
 within a single ``btf_ext_info_sec`` describe relocations applied to
 section named by ``btf_ext_info_sec->sec_name_off``.
 
-See :ref:`Documentation/bpf/llvm_reloc <btf-co-re-relocations>`
+See :ref:`Documentation/bpf/llvm_reloc.rst <btf-co-re-relocations>`
 for more information on CO-RE relocations.
 
 4.2 .BTF_ids section

Documentation/bpf/llvm_reloc.rst

@@ -250,7 +250,7 @@ CO-RE Relocations
 From object file point of view CO-RE mechanism is implemented as a set
 of CO-RE specific relocation records. These relocation records are not
 related to ELF relocations and are encoded in .BTF.ext section.
-See :ref:`Documentation/bpf/btf <BTF_Ext_Section>` for more
+See :ref:`Documentation/bpf/btf.rst <BTF_Ext_Section>` for more
 information on .BTF.ext structure.
 
 CO-RE relocations are applied to BPF instructions to update immediate