From: Daniel McNeil <daniel@osdl.org>

This adds i_seqcount to the inode structure and then uses i_size_read() and
i_size_write() to provide atomic access to i_size.  This is a port of
Andrea Arcangeli's i_size atomic access patch from 2.4.  This only uses the
generic reader/writer consistent mechanism.

Before:
mnm:/usr/src/25> size vmlinux
   text    data     bss     dec     hex filename
2229582 1027683  162436 3419701  342e35 vmlinux

After:
mnm:/usr/src/25> size vmlinux
   text    data     bss     dec     hex filename
2225642 1027655  162436 3415733  341eb5 vmlinux

3.9k more text, a lot of it fastpath :(

It's a very minor bug, and the fix has a fairly non-minor cost.  The most
compelling reason for fixing this is that writepage() checks i_size.  If it
sees a transient value it may decide that page is outside i_size and will
refuse to write it.  Lost user data.