GFS2: Add explanation of extended attr on-disk format

Some useful info regarding the on-disk representation of
GFS2 extended attributes.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

include/linux/gfs2_ondisk.h

@@ -333,6 +333,28 @@ struct gfs2_leaf {
 
 /*
  * Extended attribute header format
+ *
+ * This works in a similar way to dirents. There is a fixed size header
+ * followed by a variable length section made up of the name and the
+ * associated data. In the case of a "stuffed" entry, the value is
+ * inline directly after the name, the ea_num_ptrs entry will be
+ * zero in that case. For non-"stuffed" entries, there will be
+ * a set of pointers (aligned to 8 byte boundary) to the block(s)
+ * containing the value.
+ *
+ * The blocks containing the values and the blocks containing the
+ * extended attribute headers themselves all start with the common
+ * metadata header. Each inode, if it has extended attributes, will
+ * have either a single block containing the extended attribute headers
+ * or a single indirect block pointing to blocks containing the
+ * extended attribure headers.
+ *
+ * The maximim size of the data part of an extended attribute is 64k
+ * so the number of blocks required depends upon block size. Since the
+ * block size also determines the number of pointers in an indirect
+ * block, its a fairly complicated calculation to work out the maximum
+ * number of blocks that an inode may have relating to extended attributes.
+ *
  */
 
 #define GFS2_EA_MAX_NAME_LEN	255