9p: add support for sticky bit

GDM gets unhappy if /var/gdm doesn't have the sticky bit set.  This patch adds
support for the sticky bit in much the same way setuid/setgid is supported.

With this patch, I can launch X from a v9fs rootfs (although I quickly run out
of fds in the server once gnome starts up).
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/vfs_inode.c

@@ -77,6 +77,8 @@ static int unixmode2p9mode(struct v9fs_session_info *v9ses, int mode)
 			res |= P9_DMSETUID;
 		if ((mode & S_ISGID) == S_ISGID)
 			res |= P9_DMSETGID;
+		if ((mode & S_ISVTX) == S_ISVTX)
+			res |= P9_DMSETVTX;
 		if ((mode & P9_DMLINK))
 			res |= P9_DMLINK;
 	}
@@ -119,6 +121,9 @@ static int p9mode2unixmode(struct v9fs_session_info *v9ses, int mode)
 
 		if ((mode & P9_DMSETGID) == P9_DMSETGID)
 			res |= S_ISGID;
+
+		if ((mode & P9_DMSETVTX) == P9_DMSETVTX)
+			res |= S_ISVTX;
 	}
 
 	return res;

include/net/9p/9p.h

@@ -124,6 +124,7 @@ enum {
 	P9_DMSOCKET = 0x00100000,
 	P9_DMSETUID = 0x00080000,
 	P9_DMSETGID = 0x00040000,
+	P9_DMSETVTX = 0x00010000,
 };
 
 /* qid.types */