GFS2: Add explanation of extended attr on-disk format

/*

 * 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