lib/test_bitmap.c: fix bitmap optimisation tests to report errors correctly

I had neglected to increment the error counter when the tests failed,
which made the tests noisy when they fail, but not actually return an
error code.

Link: http://lkml.kernel.org/r/20180509114328.9887-1-mpe@ellerman.id.au
Fixes: 3cc78125

 ("lib/test_bitmap.c: add optimisation tests")
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Yury Norov <ynorov@caviumnetworks.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: <stable@vger.kernel.org>	[4.13+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>