Revert "proc: Convert proc_mount to use mount_ns."

This reverts commit e94591d0.

This cleanup broke the parsing of procfs mount parameters.

Bug: 79705088
Change-Id: I0f07180ef9a994c884abfa269ffb273ee0bcbc0d
Signed-off-by: Alistair Strachan <astrachan@google.com>