BACKPORT: FROMLIST: dm-verity: move hash algorithm setup into its own function

Move the code that sets up the hash transformation into its own
function.  No change in behavior.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

Bug: 330611177
Link: https://lore.kernel.org/r/20240621165922.77672-8-ebiggers@kernel.org


(resolved conflict due to missing the upstream commit
 "dm-verity: Convert from tasklet to BH workqueue", which is hard to
 cherry-pick because it depends on workqueue subsystem changes)
Change-Id: Ibe0a921f700bdb2de7922917e0d6cd879c042d81
Signed-off-by: Eric Biggers <ebiggers@google.com>