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.