digsig: remove registration as secondary security module

The kernel no longer supports registering a secondary security module.
This patch removes a mod_reg_security() call from digsig_init_module().

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>
digsig: rename file_mmap in digsig_security_ops

security_operations member file_mmap has been renamed to mmap_file.
This patch updates digsig_security_ops to use the new name.

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>