ocfs2: fiemap support

Plug ocfs2 into ->fiemap. Some portions of ocfs2_get_clusters() had to be
refactored so that the extent cache can be skipped in favor of going
directly to the on-disk records. This makes it easier for us to determine
which extent is the last one in the btree. Also, I'm not sure we want to be
caching fiemap lookups anyway as they're not directly related to data
read/write.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: ocfs2-devel@oss.oracle.com
Cc: linux-fsdevel@vger.kernel.org