From: "Milton D. Miller II" <miltonm@realtime.net>

The code at present has a small problem: when a fault is encountered we will
run commit_write() to cover the amount of data which was successfully copied
in from userspace.

But filemap_copy_from_user() may have zeroed out some more of the page.  So
pagecache now has zeroes and the buffer_head which represents those zeroes is
not dirtied.  So a subsequent eviction and re-read of the file in the window
beyond the faulting offset will return the file's old contents and not the
zeroes.

So we change filemap_copy_from_user_iovec() to have the same behaviour as the
non-iovec filemap_copy_from_user(), and ensure that the commit_write() covers
the parts of the page which copy_from_user() zeroed out.