bpf: Preserve inner map identity in callback frames

Callback frame constructors initialize map-typed argument registers with
__mark_reg_known_zero() and then restore map_ptr. This clears map_uid,
which is the only field distinguishing inner maps that share an
inner_map_meta template.

When a timer callback invokes bpf_for_each_map_elem() on a second inner
map, both the saved first map and the second map value can reach the nested
callback as the same template with map_uid zero. bpf_timer_init() then
accepts pairing the timer from the second map with the first map.

The runtime records the first map in the timer without taking a reference.
Freeing that map does not find the timer stored in the second map, so a
later timer callback dereferences the freed map.

Copy map_uid from the same caller register as map_ptr when constructing
for-each, timer/workqueue, and task-work callback arguments. The existing
identity check can then reject mismatched inner maps while allowing a
callback value to be paired with its actual map.

Fixes: 3e8ce29850f1 ("bpf: Prevent pointer mismatch in bpf_timer_init.")
Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Fixes: 5c8fd7e2b5b0 ("bpf: bpf task work plumbing")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904104203.345917-8-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>