dm crypt: optionally support discard requests

Add optional parameter field to dmcrypt table and support
"allow_discards" option.

Discard requests bypass crypt queue processing. Bio is simple remapped
to underlying device.

Note that discard will be never enabled by default because of security
consequences.  It is up to the administrator to enable it for encrypted
devices.

(Note that userspace cryptsetup does not understand new optional
parameters yet.  Support for this will come later.  Until then, you
should use 'dmsetup' to enable and disable this.)

Change-Id: I023bdddd5b5bc8e5abdb78d43a0bf0515e5209dc
Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>