xfs: support a tag-based inode_ag_iterator

Genericize xfs_inode_ag_walk() to support an optional radix tree tag
and args argument for the execute function. Create a new wrapper
called xfs_inode_ag_iterator_tag() that performs a tag based walk
of perag's and inodes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>