af_alg_sendmsg() takes data-to-be-copied that's provided by write(),
send(), sendmsg() and similar into pages that it allocates and will merge
new data into the last page in the list, based on the value of ctx->merge.

Now that af_alg_sendmsg() accepts MSG_SPLICE_PAGES, it adds spliced pages
directly into the list and then incorrectly appends data to them if there's
space left because ctx->merge says that it can.  This was cleared by
af_alg_sendpage(), but that got lost.

Fix this by skipping the merge if MSG_SPLICE_PAGES is specified and
clearing ctx->merge after MSG_SPLICE_PAGES has added stuff to the list.

Fixes: bf63e250 ("crypto: af_alg: Support MSG_SPLICE_PAGES")
Reported-by: Ondrej Mosnáček <omosnacek@gmail.com>
Link: https://lore.kernel.org/r/CAAUqJDvFuvms55Td1c=XKv6epfRnnP78438nZQ-JKyuCptGBiQ@mail.gmail.com/Signed-off-by: David Howells <dhowells@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Paolo Abeni <pabeni@redhat.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>