sd: Fail discard requests when logical block provisioning has been disabled

Ensure that we kill discard requests after logical block provisioning
has been disabled in sysfs.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>