Merge branch 'bpf: fix the crash caused by task iterators over vma'

Kui-Feng Lee says:

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This issue is related to task iterators over vma. A system crash can
occur when a task iterator travels through vma of tasks as the death
of a task will clear the pointer to its mm, even though the
task_struct is still held. As a result, an unexpected crash happens
due to a null pointer. To address this problem, a reference to mm is
kept on the iterator to make sure that the pointer is always
valid. This patch set provides a solution for this crash by properly
referencing mm on task iterators over vma.

The major changes from v1 are:

 - Fix commit logs of the test case.

 - Use reverse Christmas tree coding style.

 - Remove unnecessary error handling for time().

v1: https://lore.kernel.org/bpf/20221216015912.991616-1-kuifeng@meta.com/


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Signed-off-by: Alexei Starovoitov <ast@kernel.org>