ext4: add indication of ro vs r/w mounts in the mount message

ext4: add indication of ro vs r/w mounts in the mount message

Whether the file system is mounted read-only or read/write is more
important than the quota mode, which we are already printing.  Add the
ro vs r/w indication since this can be helpful in debugging problems
from the console log.

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