objtool: Fix false-positive "ignoring unreachables" warning

There's no need to try to automatically disable unreachable warnings if
they've already been manually disabled due to CONFIG_KCOV quirks.

This avoids a spurious warning with a KCOV kernel:

  fs/smb/client/cifs_unicode.o: warning: objtool: cifsConvertToUTF16.part.0+0xce5: ignoring unreachables due to jump table quirk

Fixes: eeff7ac6 ("objtool: Warn when disabling unreachable warnings")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/5eb28eeb6a724b7d945a961cfdcf8d41e6edf3dc.1744238814.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/r/202504090910.QkvTAR36-lkp@intel.com/