cifs: fix check of rc in function generate_smb3signingkey

cifs: fix check of rc in function generate_smb3signingkey

Remove extra check after condition, add check after generating key
for encryption. The check is needed to return non zero rc before
rewriting it with generating key for decryption.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Fixes: d70e9fa55884 ("cifs: try opening channels after mounting")
Signed-off-by: Ekaterina Esina <eesina@astralinux.ru>
Co-developed-by: Anastasia Belova <abelova@astralinux.ru>
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Signed-off-by: Steve French <stfrench@microsoft.com>