Add support in debugfs's set_field command to specify array specifiers for

Add support in debugfs's set_field command to specify array specifiers for
fields to be set.  For example, to set i_block[2] for the resize inode to
be 42, simply issue the debugfs command:

	set_inode <7> block[2] 42

Also added is the virtual field "bmap", which also takes an array index.
So to set the physical block mapping for logical block 1282 in the file
/test/inode to be 57, use the debugfs command:

	set_inode /test/inode bmap[1282] 57