[XFS] Fix an inode use-after-free durin an unpin. When reclaiming inodes

that have been unlinked, we may need to execute transactions during
reclaim. By the time the transaction has hit the disk, the linux inode and
xfs vnode may already have been freed so we can't reference them safely.
Use the known xfs inode state to determine if it is safe to reference the
vnode and linux inode during the unpin operation.

SGI-PV: 946321
SGI-Modid: xfs-linux-melb:xfs-kern:25687a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>