mm/gup_benchmark: add a missing "w" to getopt string

Even though gup_benchmark.c has code to handle the -w command-line option,
the "w" is not part of the getopt string.  It looks as if it has been
missing the whole time.

On my machine, this leads naturally to the following predictable result:

  $ sudo ./gup_benchmark -w
  ./gup_benchmark: invalid option -- 'w'

...which is fixed with this commit.

Link: http://lkml.kernel.org/r/20191014184639.1512873-2-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: kbuild test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>