mm/damon/lru_sort: handle ctx allocation failure

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

mm/damon/lru_sort: handle ctx allocation failure

DAMON_LRU_SORT allocates the damon_ctx object for its kdamond in its init

function. damon_lru_sort_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.