selftests/vm: gup_test: fix test flag

In gup_test both gup_flags and test_flags use the same flags field.
This is broken.

Farther, in the actual gup_test.c all the passed gup_flags are erased
and unconditionally replaced with FOLL_WRITE.

Which means that test_flags are ignored, and code like this always
performs pin dump test:

155  			if (gup->flags & GUP_TEST_FLAG_DUMP_PAGES_USE_PIN)
156  				nr = pin_user_pages(addr, nr, gup->flags,
157  						    pages + i, NULL);
158  			else
159  				nr = get_user_pages(addr, nr, gup->flags,
160  						    pages + i, NULL);
161  			break;

Add a new test_flags field, to allow raw gup_flags to work.  Add a new
subcommand for DUMP_USER_PAGES_TEST to specify that pin test should be
performed.

Remove unconditional overwriting of gup_flags via FOLL_WRITE.  But,
preserve the previous behaviour where FOLL_WRITE was the default flag,
and add a new option "-W" to unset FOLL_WRITE.

Rename flags with gup_flags.

With the fix, dump works like this:

  root@virtme:/# gup_tes...