cifs: don't leak -ENOMEM in smb2_open_file()

A NULL error response might be a valid case where smb2_reconnect()
failed to reconnect the session and tcon due to a disconnected server
prior to issuing the I/O operation, so don't leak -ENOMEM to userspace
on such occasions.

Fixes: 76894f3e ("cifs: improve symlink handling for smb2+")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>