ext4: Don't update superblock write time when filesystem is read-only

This avoids updating the superblock write time when we are mounting
the root file system read/only but we need to replay the journal; at
that point, for people who are east of GMT and who make their clock
tick in localtime for Windows bug-for-bug compatibility, and this will
cause e2fsck to complain and force a full file system check.

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