[XFS] Use the inode tree for finding dirty inodes

Update xfs_sync_inodes to walk the inode radix tree cache to find dirty
inodes. This removes a huge bunch of nasty, messy code for traversing the
mount inode list safely and removes another user of the mount inode list.

Version 3 o rediff against new linux-2.6/xfs_sync.c code

Version 2 o add comment explaining use of gang lookups for a single inode
o use IRELE, not VN_RELE o move check for ag initialisation to caller.

SGI-PV: 988139

SGI-Modid: xfs-linux-melb:xfs-kern:32290a
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>