crypto: shash - remove shash_desc::flags

The flags field in 'struct shash_desc' never actually does anything.
The only ostensibly supported flag is CRYPTO_TFM_REQ_MAY_SLEEP.
However, no shash algorithm ever sleeps, making this flag a no-op.

With this being the case, inevitably some users who can't sleep wrongly
pass MAY_SLEEP.  These would all need to be fixed if any shash algorithm
actually started sleeping.  For example, the shash_ahash_*() functions,
which wrap a shash algorithm with the ahash API, pass through MAY_SLEEP
from the ahash API to the shash API.  However, the shash functions are
called under kmap_atomic(), so actually they're assumed to never sleep.

Even if it turns out that some users do need preemption points while
hashing large buffers, we could easily provide a helper function
crypto_shash_update_large() which divides the data into smaller chunks
and calls crypto_shash_update() and cond_resched() for each chunk.  It's
not necessary to have a flag in 'struct shash_desc', nor is it necessary
to make individual shash algorithms aware of this at all.

Therefore, remove shash_desc::flags, and document that the
crypto_shash_*() functions can be called from any context.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

security/integrity/ima/ima_crypto.c

@@ -333,7 +333,6 @@ static int ima_calc_file_hash_tfm(struct file *file,
 	SHASH_DESC_ON_STACK(shash, tfm);
 
 	shash->tfm = tfm;
-	shash->flags = 0;
 
 	hash->length = crypto_shash_digestsize(tfm);
 
@@ -469,7 +468,6 @@ static int ima_calc_field_array_hash_tfm(struct ima_field_data *field_data,
 	int rc, i;
 
 	shash->tfm = tfm;
-	shash->flags = 0;
 
 	hash->length = crypto_shash_digestsize(tfm);
 
@@ -591,7 +589,6 @@ static int calc_buffer_shash_tfm(const void *buf, loff_t size,
 	int rc;
 
 	shash->tfm = tfm;
-	shash->flags = 0;
 
 	hash->length = crypto_shash_digestsize(tfm);
 
@@ -664,7 +661,6 @@ static int __init ima_calc_boot_aggregate_tfm(char *digest,
 	SHASH_DESC_ON_STACK(shash, tfm);
 
 	shash->tfm = tfm;
-	shash->flags = 0;
 
 	rc = crypto_shash_init(shash);
 	if (rc != 0)