Allow re-mounting an existing mount with "remount"

Without this line of code, if a volume is present in /proc/mounts,
it cannot be remounted with new mount options.

Docker-DCO-1.1-Signed-off-by: Peter Waller <p@pwaller.net> (github: pwaller)
Merge pull request #6669 from pwaller/issue-6652

Allow re-mounting an existing mount with "remount"