Btrfs: fix deadlock when using free space tree due to block group creation

When modifying the free space tree we can end up COWing one of its extent
buffers which in turn might result in allocating a new chunk, which in
turn can result in flushing (finish creation) of pending block groups. If
that happens we can deadlock because creating a pending block group needs
to update the free space tree, and if any of the updates tries to modify
the same extent buffer that we are COWing, we end up in a deadlock since
we try to write lock twice the same extent buffer.

So fix this by skipping pending block group creation if we are COWing an
extent buffer from the free space tree. This is a case missed by commit
5ce55557 ("Btrfs: fix deadlock when writing out free space caches").

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=202173


Fixes: 5ce55557 ("Btrfs: fix deadlock when writing out free space caches")
CC: stable@vger.kernel.org # 4.18+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>