libdm: Fix race conditions in LoopControl::Attach.

This fixes two race conditions in LoopControl::Attach(). The first is
that after LOOP_CTL_GET_FREE, the path is not be available until it has
been processed by ueventd. This can be fixed by adding a timeout
parameter and a call to WaitForFile().

Second, it is possible (albeit unlikely), given that loop devices are
now being used more aggressively, that two processes race when
attempting LOOP_SET_FD. In this case, one process will win, and the
other will fail with EBUSY. We can handle this case by retrying the
operation while respecting the same timeout parameter.

Bug: 135771280
Test: libdm_test gtest
Change-Id: Icf9facc3ca28fdb6ff5c78612d3dc183fa47b1f3