remount: Prevent error spam when remounting fails.

Cuttlefish's combined fstab has two entries for every partition, which
causes a lot of error spam when remount fails. Fix this by only
remounting entries that match an actual mount point (if such a mount
point exists).

Bug: 241179247
Test: remount on broken kernel
Change-Id: I3ddab553706f98b45f83221fd195f481dfdcc5c0