9p: define session flags

Create more general flags field in the v9fs_session_info struct and move the
'extended' flag as a bit in the flags.

Signed-off-by: Latchesar Ionkov <lucho@ionkov.net>
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/vfs_inode.c
Simple merge