mm/damon/sysfs: set damon_ctx->min_sz_region only for paddr use case

mm/damon/sysfs: set damon_ctx->min_sz_region only for paddr use case

damon_ctx->addr_unit is respected only for physical address space
monitoring use case.  Meanwhile, damon_ctx->min_sz_region is used by the
core layer for aligning regions, regardless of whether it is set for
physical address space monitoring or virtual address spaces monitoring.
And it is set as 'DAMON_MIN_REGION / damon_ctx->addr_unit'.  Hence, if
user sets ->addr_unit on virtual address spaces monitoring mode, regions
can be unexpectedly aligned in <PAGE_SIZE granularity.  It shouldn't cause
crash-like issues but make monitoring and DAMOS behavior difficult to
understand.

Fix the unexpected behavior by setting ->min_sz_region only when it is
configured for physical address space monitoring.

The issue was found from a result of Chris' experiments that thankfully
shared with me off-list.

Link: https://lkml.kernel.org/r/20250917160041.53187-1-sj@kernel.org
Fixes: d8f867fa0825 ("mm/damon: add damon_ctx->min_sz_region")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: ze zuo <zuoze1@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>