xfs: support write stream separation

xfs: support write stream separation

Allow picking a write stream ID per "active zone" equivalent on
conventional devices.  The only complicated part is stealing yet
another time stamp on the rmap inode to store the write stream
ID so we can restart after a remount without de-synchronizing the
software write pointer and the hardware equivalent.  Due to the
lack of a block layer API to query or resync our write pointer
this still can happen on power fail or a kernel crash
unfortunately.

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