Ignore functionfs mounts.

Vendors may add additional functionfs mounts. Since these
will never be remounted, we can safely filter these out.

Bug: 153008210
Test: Test device with previously unfiltered entries.
Change-Id: I7f384b8a0ce93dd6701fe3c4d9dd2557370b31e1

fs_mgr/tests/adb-remount-test.sh

@@ -732,6 +732,7 @@ skip_administrative_mounts() {
   grep -v \
     -e "^\(overlay\|tmpfs\|none\|sysfs\|proc\|selinuxfs\|debugfs\|bpf\) " \
     -e "^\(binfmt_misc\|cg2_bpf\|pstore\|tracefs\|adb\|mtp\|ptp\|devpts\) " \
+    -e " functionfs " \
     -e "^\(/data/media\|/dev/block/loop[0-9]*\) " \
     -e "^rootfs / rootfs rw," \
     -e " /\(cache\|mnt/scratch\|mnt/vendor/persist\|persist\|metadata\) "