cifs: remove redundant initialization to variable mnt_sign_enabled

Variable mnt_sign_enabled is being initialized with a value that
is never read, it is being reassigned later on with a different
value. The initialization is redundant and can be removed.

Cleans up clang scan-build warning:
fs/cifs/cifssmb.c:465:7: warning: Value stored to 'mnt_sign_enabled
 during its initialization is never read

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>