In v4 we grab an extra page just for the padding of returned data.  The
formula that the rpc server uses to allocate pages for the response doesn't
take into account this extra page.

Instead of adjusting those formulae, we adopt the same solution as v2 and v3,
and put the "tail" data in the same page as the "head" data.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NNeil Brown <neilb@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>