cifs: Fix reversion of the iter in cifs_readv_receive().

cifs_read_iter_from_socket() copies the iterator that's passed in for the
socket to modify as and if it will, and then advances the original iterator
by the amount sent.  However, both callers revert the advancement (although
receive_encrypted_read() zeros beyond the iterator first).  The problem is,
though, that cifs_readv_receive() reverts by the original length, not the
amount transmitted which can cause an oops in iov_iter_revert().

Fix this by:

 (1) Remove the iov_iter_advance() from cifs_read_iter_from_socket().

 (2) Remove the iov_iter_revert() from both callers.  This fixes the bug in
     cifs_readv_receive().

 (3) In receive_encrypted_read(), if we didn't get back as much data as the
     buffer will hold, copy the iterator, advance the copy and use the copy
     to drive iov_iter_zero().

As a bonus, this gets rid of some unnecessary work.

This was triggered by generic/074 with the "-o sign" mount option.

Fixes: 3ee1a1fc ("cifs: Cut over to using netfslib")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>