mm/damon/reclaim: handle ctx allocation failure

In the Linux kernel, the following vulnerability has been resolved:

mm/damon/reclaim: handle ctx allocation failure

Patch series "mm/damon/{reclaim,lru_sort}: handle ctx allocation failures".

DAMON_RECLAIM and DAMON_LRU_SORT could dereference NULL pointers if their
damon_ctx object allocations fail. The bugs are expected to happen
infrequently because the allocations are arguably too small to fail on
common setups. But theoretically they are possible and the consequences
are bad. Fix those.

The issues were discovered [1] by Sashiko.

This patch (of 2):

DAMON_RECLAIM allocates the damon_ctx object for its kdamond in its init
function. damon_reclaim_enabled_store() wrongly assumes the allocation
will always succeed once tried. If the damon_ctx allocation was failed,
therefore, code execution reaches to damon_commit_ctx() while 'ctx' is
NULL. As a result, it dereferences the NULL 'ctx' pointer. Avoid the
NULL dereference by returning -ENOMEM if 'ctx' is NULL.