fsverity: store log2(digest_size) precomputed

Add log_digestsize to struct merkle_tree_params so that it can be used
in verify.c.  Also save memory by using u8 for all the log_* fields.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221223203638.41293-4-ebiggers@kernel.org