fix kABI breakage from "blk-mq: Shared tag enhancements"

fix kABI breakage from "blk-mq: Shared tag enhancements"

Orabug: 23521058

Commit e92f419 ("blk-mq: Shared tag enhancements") added the element
"cpumask" to struct blk_mq_tags.  This broke kABI.

Currently, this structure is only used by the core blk-mq code.
I don't expect that third-party modules will use it at all.
Even if they do, though, the kernel provides an API that
encapsulates creation/destruction/access for these structures,
and modules that use that API will be unaffected by the change.

So the workaround for the kABI breakage is simply to hide the new
element from the kABI checker by wrapping it in #ifndef __GENKSYMS__.

Signed-off-by: Dan Duval <dan.duval@oracle.com>