In order to help the compiler reason about the destination buffer in struct
fc_nl_event, add a flexible array member for this purpose.  However, since
the header is UAPI, it must not change size or layout, so a union is used.

The allocation size calculations are also corrected (it was potentially
allocating an extra 8 bytes), and the padding is zeroed to avoid leaking
kernel heap memory contents.

Detected at run-time by the recently added memcpy() bounds checking:

  memcpy: detected field-spanning write (size 8) of single field "&event->event_data" at drivers/scsi/scsi_transport_fc.c:581 (size 4)

Link: https://lore.kernel.org/linux-next/42404B5E-198B-4FD3-94D6-5E16CF579EF3@linux.ibm.com/
Link: https://lore.kernel.org/r/20220921205155.1451649-1-keescook@chromium.org
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Reviewed-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>