[XFS] Ondisk format extension for extended attributes (attr2). Basically,

the data/attr forks now grow up/down from either end of the literal area,
rather than dividing the literal area into two chunks and growing both
upward.  Means we can now make much more efficient use of the attribute
space, incl. fitting DMF attributes inline in 256 byte inodes, and large
jumps in dbench3 performance numbers.  It is self enabling, but can be
forced on/off via the attr2/noattr2 mount options.

SGI-PV: 941645
SGI-Modid: xfs-linux:xfs-kern:23837a
Signed-off-by: Nathan Scott <nathans@sgi.com>

fs/xfs/xfs_mount.h

@@ -404,7 +404,7 @@ typedef struct xfs_mount {
 						   user */
 #define XFS_MOUNT_NOALIGN	0x00000080	/* turn off stripe alignment
 						   allocations */
-			     /* 0x00000100	-- currently unused */
+#define XFS_MOUNT_COMPAT_ATTR	0x00000100	/* do not use attr2 format */
 			     /*	0x00000200	-- currently unused */
 #define XFS_MOUNT_NORECOVERY	0x00000400	/* no recovery - dirty fs */
 #define XFS_MOUNT_SHARED	0x00000800	/* shared mount */