linux/concepts: fix a mistake on atomics return value

See more details in the specification[^0]:
> The BPF_FETCH modifier is optional for simple atomic operations, and
always set for the complex atomic operations. If the BPF_FETCH flag is
set, then the operation also overwrites src with the value that was in
memory before it was modified.

[^0]: https://www.kernel.org/doc/html/v6.5/bpf/instruction-set.html#atomic-operations

Signed-off-by: Mahe Tardy <[email protected]>

docs/linux/concepts/concurrency.md

@@ -22,10 +22,10 @@ Since this happens in multiple steps, even such a simple operation is subject to
 
 There is a class of CPU instructions that can perform specific tasks in a single CPU instruction which is serialized at the hardware level. These are also available in BPF. When compiling with Clang/LLVM these special instructions can be accessed via a list of special builtin functions:
 
-* `__sync_fetch_and_add(*a, b)` - Read value at `a`, add `b` and write it back, return the new value
-* `__sync_fetch_and_sub(*a, b)` - Read value at `a`, subtract a number and write it back, return the new value
-* `__sync_fetch_and_or(*a, b)` - Read value at `a`, binary OR a number and write it back, return the new value
-* `__sync_fetch_and_xor(*a, b)` - Read value at `a`, binary XOR a number and write it back, return the new value
+* `__sync_fetch_and_add(*a, b)` - Read value at `a`, add `b` and write it back, return the original value of `a`
+* `__sync_fetch_and_sub(*a, b)` - Read value at `a`, subtract a number and write it back, return the original value of `a`
+* `__sync_fetch_and_or(*a, b)` - Read value at `a`, binary OR a number and write it back, return the original value of `a`
+* `__sync_fetch_and_xor(*a, b)` - Read value at `a`, binary XOR a number and write it back, return the original value of `a`
 * `__sync_val_compare_and_swap(*a, b, c)` - Read value at `a`, check if it is equal to `b`, if true write `c` to `a` and return the original value of `a`. On fail leave `a` be and return `c`.
 * `__sync_lock_test_and_set(*a, b)` - Read value at `a`, write `b` to `a`, return original value of `a`