crypto: hash - Fix test underflow in shash_ahash_digest

crypto: hash - Fix test underflow in shash_ahash_digest

The test on PAGE_SIZE - offset in shash_ahash_digest can underflow,
leading to execution of the fast path even if the data cannot be
mapped into a single page.

Fix this by splitting the test into four cases:

1) nbytes > sg->length: More than one SG entry, slow path.
2) !IS_ENABLED(CONFIG_HIGHMEM): fast path.
3) nbytes > (unsigned int)PAGE_SIZE - offset: Two highmem pages, slow path.
4) Highmem fast path.

Fixes: 5f7082ed4f48 ("crypto: hash - Export shash through hash")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>