Store XfrmDirection in XfrmSpInfo

XfrmDirection and XfrmSpInfo are always used together. This commit
stores the XfrmDirection in XfrmSpInfo to make the code cleaner.
This commit is also a preparation for the following CL to add
XfrmMigrate support.

Bug: 169170985
Test: atest netd_integration_test
Change-Id: I4b9a6778fc09b3e473336d2f8b9053c98950bfd2
2 files changed