locking/atomics: Change 'fold' to 'grep'

Some distibutions and build systems doesn't include 'fold' from
coreutils default.

.../scripts/atomic/atomic-tbl.sh: line 183: fold: command not found

Rework to use 'grep' instead of 'fold' to use a dependency that is
already used a lot in the kernel.

[Mark: rework commit message]

Suggested-by: Will Deacon <will.deacon@arm.com>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Cc: boqun.feng@gmail.com
Cc: linux-kernel@vger.kernel.rg
Signed-off-by: Ingo Molnar <mingo@kernel.org>