xfs: fix panic_mask documentation

This bit of the docs didn't quite reflect reality.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

Documentation/filesystems/xfs.txt

@@ -287,9 +287,9 @@ The following sysctls are available for the XFS filesystem:
 		XFS_ERRLEVEL_LOW:       1
 		XFS_ERRLEVEL_HIGH:      5
 
-  fs.xfs.panic_mask		(Min: 0  Default: 0  Max: 127)
+  fs.xfs.panic_mask		(Min: 0  Default: 0  Max: 255)
 	Causes certain error conditions to call BUG(). Value is a bitmask;
-	AND together the tags which represent errors which should cause panics:
+	OR together the tags which represent errors which should cause panics:
 
 		XFS_NO_PTAG                     0
 		XFS_PTAG_IFLUSH                 0x00000001
@@ -299,6 +299,7 @@ The following sysctls are available for the XFS filesystem:
 		XFS_PTAG_SHUTDOWN_CORRUPT       0x00000010
 		XFS_PTAG_SHUTDOWN_IOERROR       0x00000020
 		XFS_PTAG_SHUTDOWN_LOGERROR      0x00000040
+		XFS_PTAG_FSBLOCK_ZERO           0x00000080
 
 	This option is intended for debugging only.