zero out last page for llseek/write

When one llseek's past the end of the file and then writes, every page past
the previous end of the file should be cleared.  Trevor found that the code,
as is, does not assure that the very last page is always cleared.  This patch
takes care of that.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>