fs_mgr: overlayfs: validate that kernel supports scratch filesystem

Check for /sys/fs/<fs_type> existence to confirm kernel has
the file system type available.

Test: adb-remount-test.sh
Bug: 109821005
Bug: 123079041
Change-Id: Ic4388e2044bccea8b8edc7762d4ac9b3047206ac
1 file changed