Add support for recent kmem_cache_create_usercopy

Add support for recent kmem_cache_create_usercopy

SLAB_USERCOPY flag was used to indicate PAX
not to kill copies from kernel to userland.

With recent grsecurity patchset and
CONFIG_GRKERNSEC_HIDESYM that enables
CONFIG_PAX_USERCOPY zfs would panic.

Handle newer API while keeping old one functional.

Tested-by: RageLtMan <rageltman@sempervictus>
Reviewed-by: spendergrsec <spender@grsecurity.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kevin Tanguy <kevin.tanguy@ovh.net>
Closes #595