fs-verity: don't pass whole descriptor to fsverity_verify_signature()

fs-verity: don't pass whole descriptor to fsverity_verify_signature()

Now that fsverity_get_descriptor() validates the sig_size field,
fsverity_verify_signature() doesn't need to do it.

Just change the prototype of fsverity_verify_signature() to take the
signature directly rather than take a fsverity_descriptor.

Link: https://lore.kernel.org/r/20210115181819.34732-3-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Amy Parker <enbyamy@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>