e2fsck: avoid unnecessary reboots in some cases when checking the root fs

e2fsck: avoid unnecessary reboots in some cases when checking the root fs

If e2fsck modifies certain superblock fields which the kernel doesn't
look at, mark the superblock as dirty without marking the file system
as changed.  This will avoid e2fsck signalling the init scripts that a
reboot is necessary.  This is safe, because the kernel doesn't
actually look at these superblock fields.

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