9P: introduction of a new cache=mmap model.

 - Add cache=mmap option
 - Make mmap read-write while keeping it as synchronous as possible
 - Build writeback fid on mmap creation if it is writable

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

diff --cc fs/9p/v9fs.c
Simple merge

diff --cc fs/9p/v9fs.h
Simple merge

diff --cc fs/9p/v9fs_vfs.h
Simple merge

diff --cc fs/9p/vfs_addr.c
Simple merge

diff --cc fs/9p/vfs_file.c
Simple merge

diff --cc fs/9p/vfs_inode.c
Simple merge

diff --cc fs/9p/vfs_inode_dotl.c
Simple merge

diff --cc fs/9p/vfs_super.c
Simple merge