error-injection: support static keys around injectable functions

Error injectable functions cannot be inlined and since some are called
from hot paths, this incurrs overhead even if no error injection is
enabled for them.

To remove this overhead when disabled, allow the callsites of error
injectable functions to put the calls behind a static key, which the
framework can control when error injection is enabled or disabled for
the function.

Introduce a new ALLOW_ERROR_INJECTION_KEY() macro that adds a parameter
with the static key's address, and store it in struct
error_injection_entry. This new field has caused a mismatch when
populating the injection list from the _error_injection_whitelist
section with the current STRUCT_ALIGN(), so change the alignment to 8.

During the population, copy the key's address also to struct ei_entry,
and make it possible to retrieve it along with the error type by
get_injectable_error_type().

Finally, make the processing of writes to the debugfs inject file enable
the static key when the function is added to the injection list, and
disable when removed.

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