[net/9p] Preparation and helper functions for zero copy

This patch prepares p9_fcall structure for zero copy. Added
fields send the payload buffer information to the transport layer.
In addition it adds a 'private' field for the transport layer to
store mapped/pinned page information so that it can be freed/unpinned
during req_done.

This patch also creates trans_common.[ch] to house helper functions.
It adds the following helper functions.

p9_release_req_pages - Release pages after the transaction.
p9_nr_pages - Return number of pages needed to accomodate the payload.
payload_gup - Translates user buffer into kernel pages.

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
diff --git a/net/9p/Makefile b/net/9p/Makefile
index 198a640..a0874cc 100644
--- a/net/9p/Makefile
+++ b/net/9p/Makefile
@@ -9,6 +9,7 @@
 	util.o \
 	protocol.o \
 	trans_fd.o \
+	trans_common.o \
 
 9pnet_virtio-objs := \
 	trans_virtio.o \