ext4: update the s_last_mounted field in the superblock

This field can be very helpful when a system administrator is trying
to sort through large numbers of block devices or filesystem images.
What is stored in this field can be ambiguous if multiple filesystem
namespaces are in play; what we store in practice is the mountpoint
interpreted by the process's namespace which first opens a file in the
filesystem.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>