Allow the kernel to access apexd file descriptors.

In earlier kernel versions (<4.0), the loopback driver issues
requests from a kernel thread. Therefore, the kernel needs access
to APEX file descriptors and data files (which are loopback
mounted).

Bug: 119220815
Test: mounting works on sailfish
Change-Id: I75b2bade41c64cf6fa6040d9c2f5489a206e04c6