Btrfs: Add a stripe cache to raid56

The stripe cache allows us to avoid extra read/modify/write cycles
by caching the pages we read off the disk.  Pages are cached when:

* They are read in during a read/modify/write cycle

* They are written during a read/modify/write cycle

* They are involved in a parity rebuild

Pages are not cached if we're doing a full stripe write.  We're
assuming that a full stripe write won't be followed by another
partial stripe write any time soon.

This provides a substantial boost in performance for workloads that
synchronously modify adjacent offsets in the file, and for the parity
rebuild use case in general.

The size of the stripe cache isn't tunable (yet) and is set at 1024
entries.

Example on flash: dd if=/dev/zero of=/mnt/xxx bs=4K oflag=direct

Without the stripe cache  -- 2.1MB/s
With the stripe cache 21MB/s

Signed-off-by: Chris Mason <chris.mason@fusionio.com>