btrfs: fix mixed block count of available space

[ Upstream commit ae02d1bd070767e109f4a6f1bb1f466e9698a355 ]

Metadata for mixed block is already accounted in total data and should not
be counted as part of the free metadata space.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=114281


Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>