GFS2: speed up delete/unlink performance for large files

This patch improves the performance of delete/unlink
operations in a GFS2 file system where the files are large
by adding a layer of metadata read-ahead for indirect blocks.
Mileage will vary, but on my system, deleting an 8.6G file
dropped from 22 seconds to about 4.5 seconds.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>