roots: Fix an issue with volume_for_path().

The earlier commit in 2dfc1a38
unintentionally changed the behavior. It gives a different result when
looking up non-existent mount points (e.g. /cache on marlin).

The logic behind volume_for_path("/xyz") is unclear:
- It's fine to return non-null value if it's called by
  ensure_path_mounted() before accessing that file "/xyz". (Just based
  on the function name, we're not actually having this case.)
- It should return nullptr if the caller is interested in the existence
  of that particular mount point "/xyz".

This CL renames the function to volume_for_mount_point(), which does an
exact match by querying the given mount point from libfs_mgr. The former
volume_for_path() has been moved down to function scope for serving
ensure_path_mounted() only.

Test: Build and boot into recovery on bullhead and marlin respectively.
      'View recovery logs'.
Test: 'Mount /system'
Test: 'Apply update from ADB'
Change-Id: I1a16390f57540cae08a2b8f3d439d17886975217