arm64: spinlock: retry trylock operation if strex fails on free lock

An exclusive store instruction may fail for reasons other than lock
contention (e.g. a cache eviction during the critical section) so, in
line with other architectures using similar exclusive instructions
(alpha, mips, powerpc), retry the trylock operation if the lock appears
to be free but the strex reported failure.

Change-Id: Iee32ff1d1b0ef9e40e80361acbe53396719a37e1
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Tony Thompson <anthony.thompson@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Git-commit: 4ecf7ccb1973fd826456b6ab1e6dfafe9023c753
Git-Repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>