smb3: fix oops in calculating shash_setkey

shash was not being initialized in one place in smb3_calc_signature
and smb2_calc_signature

Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Acked-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Steve French <stfrench@microsoft.com>