btrfs: extent-tree: Refactor add_pinned_bytes() to add|sub_pinned_bytes()

Instead of using @sign to determine whether we're adding or subtracting.
Even it only has 3 callers, it's still (and in fact already caused
problem in the past) confusing to use.

Refactor add_pinned_bytes() to add_pinned_bytes() and sub_pinned_bytes()
to explicitly show what we're doing.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>