mm: Make failslab writable again

In (060807f841ac mm, slub: make remaining slub_debug related attributes
read-only) failslab was made read-only.
I think it became a collateral victim to the two other options for which
the reasons are perfectly valid.
Here is why:
 - sanity_checks and trace are slab internal debug options,
   failslab is used for fault injection.
 - for fault injections, which by presumption are random, it
   does not matter if it is not set atomically. And you need to
   set atleast one more option to trigger fault injection.
 - in a testing scenario you may need to change it at runtime
   example: module loading - you test all allocations limited
   by the space option. Then you move to test only your module's
   own slabs.
 - when set by command line flags it effectively disables all
   cache merges.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Jann Horn <jannh@google.com>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200610163135.17364-5-vbabka@suse.cz

Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

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