mm, slab: add static key for should_failslab()

Since commit 4f6923fbb352 ("mm: make should_failslab always available for
fault injection") should_failslab() is unconditionally a noinline
function. This adds visible overhead to the slab allocation hotpath,
even if the function is empty. With CONFIG_FAILSLAB=y there's additional
overhead when the functionality is not enabled by a boot parameter or
debugfs.

The overhead can be eliminated with a static key around the callsite.
Fault injection and error injection frameworks can now be told that the
this function has a static key associated, and are able to enable and
disable it accordingly.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>