Btrfs: reduce lock contention on extent buffer locks

The extent buffers have a refs_lock which we use to make coordinate freeing
the extent buffer with operations on the radix tree.  On tree roots and
other extent buffers that very cache hot, this can be highly contended.

These are also the extent buffers that are basically pinned in memory.
This commit adds code to cmpxchg our way through the ref modifications,
and as long as the result of the reference change is still pinned in
ram, we skip the expensive spinlock.

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