9p: embed fcall in req to round down buffer allocs

'msize' is often a power of two, or at least page-aligned, so avoiding
an overhead of two dozen bytes for each allocation will help the
allocator do its work and reduce memory fragmentation.

Link: http://lkml.kernel.org/r/1533825236-22896-1-git-send-email-asmadeus@codewreck.org
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Reviewed-by: Greg Kurz <groug@kaod.org>
Acked-by: Jun Piao <piaojun@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>