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)
 
Fix #6652

Tested to the extent that I see the syscall through strace with `remount`
and this new code but not without.