[XFS] avoid all reclaimable inodes in xfs_sync_inodes_ag

If we are syncing data in xfs_sync_inodes_ag(), the VFS inode must still
be referencable as the dirty data state is carried on the VFS inode. hence
if we can't get a reference via igrab(), the inode must be in reclaim
which implies that it has no dirty data attached.

Leave such inodes to the reclaim code to flush the dirty inode state to
disk and so avoid attempting to access the VFS inode when it may not exist
in xfs_sync_inodes_ag().

Version 4:
o don't reference linux inode until after igrab() succeeds

Version 3:
o converted unlock/rele to an xfs_iput() call.

Version 2:
o change igrab logic to be more linear
o remove initial reclaimable inode check now that we are using
  igrab() failure to find reclaimable inodes
o assert that igrab failure occurs only on reclaimable inodes
o clean up inode locking - only grab the iolock if we are doing
  a SYNC_DELWRI call and we have a dirty inode.

SGI-PV: 987246

SGI-Modid: xfs-linux-melb:xfs-kern:32391a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Peter Leckie <pleckie@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>