rust: sync: atomic: Add fetch_sub()

rust: sync: atomic: Add fetch_sub()

Add `Atomic::fetch_sub()` with implementation and documentation in line
with existing `Atomic::fetch_add()` implementation.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260220-atomic-sub-v3-1-e63cbed1d2aa@kernel.org
Link: https://patch.msgid.link/20260303201701.12204-12-boqun@kernel.org