mm/damon/vaddr: add 'nr_piece == 1' check in damon_va_evenly_split_region()

As discussed in [1], damon_va_evenly_split_region() is called to
size-evenly split a region into 'nr_pieces' small regions,
when nr_pieces == 1, no actual split is required. Check that case
for better code readability and add a simple kunit testcase.

[1] https://lore.kernel.org/all/20241021163316.12443-1-sj@kernel.org/

Link: https://lkml.kernel.org/r/20241022083927.3592237-3-zhengyejian@huaweicloud.com
Signed-off-by: Zheng Yejian <zhengyejian@huaweicloud.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Fernand Sieber <sieberf@amazon.com>
Cc: Leonard Foerster <foersleo@amazon.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Ye Weihua <yeweihua4@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Zheng Yejian

mm/damon/tests/vaddr-kunit.h

@@ -300,6 +300,7 @@ static void damon_test_split_evenly(struct kunit *test)
 	damon_test_split_evenly_fail(test, 0, 100, 0);
 	damon_test_split_evenly_succ(test, 0, 100, 10);
 	damon_test_split_evenly_succ(test, 5, 59, 5);
+	damon_test_split_evenly_succ(test, 4, 6, 1);
 	damon_test_split_evenly_succ(test, 0, 3, 2);
 	damon_test_split_evenly_fail(test, 5, 6, 2);
 }

mm/damon/vaddr.c

@@ -72,6 +72,9 @@ static int damon_va_evenly_split_region(struct damon_target *t,
 	if (!r || !nr_pieces)
 		return -EINVAL;
 
+	if (nr_pieces == 1)
+		return 0;
+
 	orig_end = r->ar.end;
 	sz_orig = damon_sz_region(r);
 	sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION);