mm/damon/core: remove damon_verify_nr_regions()

mm/damon/core: remove damon_verify_nr_regions()

When CONFIG_DAMON_DEBUG_SANITY is enabled, damon_verify_nr_regions() is
called for each damon_nr_regions() invocation.  damon_veify_nr_regions()
iterates all regions.  damon_nr_regions() is called for each region in
kdamond_reset_aggregated() and damos_apply_scheme().  Hence it imposes
O(n**2) overhead where n is the number of regions.

Though the verification is enabled only under DAMON_DEBUG_SANITY, which is
not for production use cases, it could be too high overhead.  Meanwhile,
damon_verify_ctx() is doing the damon_nr_regions() test.  Because
damon_verify_ctx() is called for each kdamond_call(), the test coverage
from damon_verify_ctx() could be sufficient.  Remove damon_nr_regions()
verification.

Link: https://lore.kernel.org/20260522154026.80546-10-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>