btrfs: load free space cache asynchronously

btrfs: load free space cache asynchronously

While documenting the usage of the commit_root_sem, I noticed that we do
not actually take the commit_root_sem in the case of the free space
cache.  This is problematic because we're supposed to hold that sem
while we're reading the commit roots, which is what we do for the free
space cache.

The reason I did it inline when I originally wrote the code was because
there's the case of unpinning where we need to make sure that the free
space cache is loaded if we're going to use the free space cache.  But
we can accomplish the same thing by simply waiting for the cache to be
loaded.

Rework this code to load the free space cache asynchronously.  This
allows us to greatly cleanup the caching code because now it's all
shared by the various caching methods.  We also are now in a position to
have the commit_root semaphore held while we're loading the free space
cache.  And finally our modification of ->last_byte_to_unpin is removed
because it can be handled in the proper way on commit.

Some care must be taken when replaying the log, when we expect that the
free space cache will be read entirely before we start excluding space
to replay. This could lead to overwriting space during replay.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>