UPSTREAM: arm64: spinlock: fix spin_unlock_wait for LSE atomics

UPSTREAM: arm64: spinlock: fix spin_unlock_wait for LSE atomics

Commit d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against
concurrent lockers") fixed spin_unlock_wait for LL/SC-based atomics under
the premise that the LSE atomics (in particular, the LDADDA instruction)
are indivisible.

Unfortunately, these instructions are only indivisible when used with the
-AL (full ordering) suffix and, consequently, the same issue can
theoretically be observed with LSE atomics, where a later (in program
order) load can be speculated before the write portion of the atomic
operation.

This patch fixes the issue by performing a CAS of the lock once we've
established that it's unlocked, in much the same way as the LL/SC code.

Fixes: d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers")
Signed-off-by: Will Deacon <will.deacon@arm.com>
Bug: 30369029
Patchset: kaslr-arm64-4.4

(cherry picked from commit 3a5facd09da848193f5bcb0dea098a298bc1a29d)
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Change-Id: Icedaa4c508784bf43d0b5787586480fd668ccc49