digsig: rename digsig_file_mmap() to digsig_mmap_file()

Member file_mmap of struct security_operations was renamed to mmap_file
a long time ago, so we can rename digsig_file_mmap() to
digsig_mmap_file() to match this change.

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