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

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/[email protected]
Fixes: 3cc78125a081 ("lib/test_bitmap.c: add optimisation tests")
Signed-off-by: Matthew Wilcox <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Reported-by: Michael Ellerman <[email protected]>
Tested-by: Michael Ellerman <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Yury Norov <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: <[email protected]>	[4.13+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>