mm/damon/core: use a dedicated struct for monitoring attributes

DAMON monitoring attributes are directly defined as fields of 'struct
damon_ctx'.  This makes 'struct damon_ctx' a little long and complicated. 
This commit defines and uses a struct, 'struct damon_attrs', which is
dedicated for only the monitoring attributes to make the purpose of the
five values clearer and simplify 'struct damon_ctx'.

Link: https://lkml.kernel.org/r/20220913174449.50645-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>