kernel: Add missing config option for kernel 4.14

Kernel 4.14.96 got the new configuration option
CIFS_ALLOW_INSECURE_LEGACY which allows to deactivate support for old
and insecure SMB versions like 1.0 and 2.0. Still allow these old SMB
version and fix build problems which occurred because this option was
not defined.

This was found by build bot.

Fixes: 3662157d8bed ("kernel: bump 4.14 to 4.14.96")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>