objtool/klp: Fix symbol correlation for orphaned local symbols

objtool/klp: Fix symbol correlation for orphaned local symbols

When compiling with CONFIG_LTO_CLANG_THIN, vmlinux.o has
__irf_[start|end] before the first FILE entry:

  $ readelf -sW vmlinux.o
  Symbol table '.symtab' contains 597706 entries:
     Num:    Value          Size Type    Bind   Vis      Ndx Name
       0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
       1: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT   18 __irf_start
       2: 0000000000000200     0 NOTYPE  LOCAL  DEFAULT   18 __irf_end
       3: 0000000000000000     0 SECTION LOCAL  DEFAULT   17 .text
       4: 0000000000000000     0 SECTION LOCAL  DEFAULT   18 .init.ramfs

This causes klp-build warnings like:

  vmlinux.o: warning: objtool: no correlation: __irf_start
  vmlinux.o: warning: objtool: no correlation: __irf_end

The problem is that Clang LTO is stripping the initramfs_data.o FILE
symbol, causing those two symbols to be orphaned and not noticed by
klp-diff's correlation logic.  Add a loop to correlate any symbols found
before the first FILE symbol.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Reported-by: Song Liu <song@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://patch.msgid.link/e21ec1141fc749b5f538d7329b531c1ab63a6d1a.1770055235.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>