tools/testing/selftests/vm/gup_benchmark.c: fix 'write' flag usage

tools/testing/selftests/vm/gup_benchmark.c: fix 'write' flag usage

If the '-w' parameter was provided, the benchmark would exit due to a
mssing 'break'.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Keith Busch <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Dan Williams <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>