False Positive return_cast check

Hello,

I hope this is the right avenue to report this.

I noticed yesterday that we get a false positive report from smatch for
`pathmask_to_pos()` in `arch/s390/include/asm/cio.h`. When reviewing
some code I got this error while using smatch:

    CHECK   /home/bblock/src/linux/drivers/s390/cio/device_pgid.c
  /home/bblock/src/linux/arch/s390/include/asm/cio.h:356 pathmask_to_pos() warn: signedness bug returning '(-119)'

Which corresponds to this helper:

  static inline u8 pathmask_to_pos(u8 mask)
  {
  	return 8 - ffs(mask);
  }

First I assumed this corresponds to some corner case where `ffs()` might
return a negative number, or something bigger than 8 (both are not
supposed to be possible); but I couldn't wrap my head around what
exactly the error message is supposed to tell me with `(-119)`. 

After some exhaustive head scratching I took a look at smatch itself and
found this message is emitted by `check_return_cast.c`, and according to
commit `f65a092d16cf` is supposed to warn me that we return a negative
error code via an unsigned return value type.
    That seems to be a false positive here.

Its not really something terrible important, but after I took so long to
understand what this is telling me, I thought I might as well report it.

-- 
Best Regards, Benjamin Block  / Linux on IBM Z Kernel Development / IBM Systems
IBM Deutschland Research & Development GmbH    /    https://www.ibm.com/privacy
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