cifs: Fix null pointer check in cifs_read

[ Upstream commit 9bd21d4b1a767c3abebec203342f3820dcb84662 ]

Coverity scan noted a redundant null check

Coverity-id: 728517
Reported-by: Coverity <scan-admin@coverity.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Shyam Prasad N <nspmangalore@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>