dm crypt: leverage immutable biovecs when decrypting on read

Commit 003b5c57 ("block: Convert drivers to immutable biovecs")
stopped short of changing dm-crypt to leverage the fact that the biovec
array of a bio will no longer be modified.

Switch to using bio_clone_fast() when cloning bios for decryption after
read.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 5977907937afa2b5584a874d44ba6c0f56aeaa9c)
Signed-off-by: Alex Shi <alex.shi@linaro.org>