cifs: use discard iterator to discard unneeded network data more efficiently

[ Upstream commit cf0604a6 ]

The iterator, ITER_DISCARD, that can only be used in READ mode and
just discards any data copied to it, was added to allow a network
filesystem to discard any unwanted data sent by a server.
Convert cifs_discard_from_socket() to use this.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Stable-dep-of: bedc8f76 ("cifs: always initialize struct msghdr smb_msg completely")
Signed-off-by: Sasha Levin <sashal@kernel.org>