smb: client: stop flooding dmesg in smb2_calc_signature()

When having several mounts that share same credential and the client
couldn't re-establish an SMB session due to an expired kerberos ticket
or rotated password, smb2_calc_signature() will end up flooding dmesg
when not finding SMB sessions to calculate signatures.

Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>