Speeds up generic_file_write() by not calling mark_inode_dirty() when
the mtime and ctime didn't change.

There may be concerns over the fact that this restricts mtime and ctime
updates to one-second resolution.  But the interface doesn't support
that anyway - all the filesystem knows is that its dirty_inode()
superop was called.  It doesn't know why.

So filesystems which support high-resolution timestamps already need to
make their own arrangements.  We need an update_mtime i_op to support
those properly.

time to write a one megabyte file one-byte-at-a-time:

Before:
	ext3:		24.8 seconds
	ext2:		 4.9 seconds
	reiserfs:	17.0 seconds
After:
	ext3:		22.5 seconds
	ext2:		4.8  seconds
	reiserfs:	11.6 seconds

Not much improvement because we're also calling expensive
mark_inode_dirty() functions when i_size is expanded.  So compare the
overwrite case:

time dd if=/dev/zero of=foo bs=1 count=1M conv=notrunc

ext3 before:	20.0 seconds
ext3 after:	9.7  seconds