make pdfdocs fails on Debian stable

[Christoph Hellwig]

Hi Jon and Mauro,

when trying to do a make pdfdocs on my Debian stable system I can't
get it to complete.  The output below is from linux-next, but mainline
is the same.

------------
Running 'xelatex --no-pdf -interaction=batchmode -no-pdf -recorder  "RCU.tex"'
------------
This is XeTeX, Version 3.14159265-2.6-0.999992 (TeX Live 2020/Debian) (preloaded format=xelatex)
restricted \write18 enabled.
 entering extended mode
Latexmk: Examining 'RCU.log'
=== TeX engine is 'XeTeX'
Latexmk: Index file 'RCU.idx' was written
Latexmk: References changed.
Latexmk: Log file says output to 'RCU.xdv'
Latexmk: Errors, so I did not complete making targets
Collected error summary (may duplicate other messages):
  xelatex: Command for 'xelatex' gave return code 1
    Refer to 'RCU.log' for details
Latexmk: Use the -f option to force complete processing,
  unless error was exceeding maximum runs, or warnings treated as errors.
make[2]: *** [Makefile:29: RCU.pdf] Error 12
make[1]: *** [Documentation/Makefile:115: pdfdocs] Error 2
make: *** [Makefile:1772: pdfdocs] Error 2