xfs: rewrite the filestream allocator using the dentry cache

In Linux we will always be able to find a parent inode for file that are
undergoing I/O.  Use this to simply the file stream allocator by only
keeping track of parent inodes.

Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --cc fs/xfs/xfs_filestream.c
Simple merge

diff --cc fs/xfs/xfs_filestream.h
Simple merge

diff --cc fs/xfs/xfs_inode.c
Simple merge