Merge branch 'Fix for crash due to overwrite in copy_map_value'

Kumar Kartikeya says:

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A fix for an oversight in copy_map_value that leads to kernel crash.

Also, a question for BPF developers:
It seems in arraymap.c, we always do check_and_free_timer_in_array after we do
copy_map_value in map_update_elem callback, but the same is not done for
hashtab.c. Is there a specific reason for this difference in behavior, or did I
miss that it happens for hashtab.c as well?

Changlog:
---------
v1 -> v2:
v1: https://lore.kernel.org/bpf/20220209051113.870717-1-memxor@gmail.com



 * Fix build error for selftests patch due to missing SYS_PREFIX in bpf tree
====================

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