btrfs: use per-profile available space in calc_available_free_space()

For the following disk layout, can_overcommit() can cause false
confidence in available space:

  devid 1 unallocated:	1GiB
  devid 2 unallocated:	50GiB
  metadata type:	RAID1

As can_overcommit() simply uses unallocated space with factor to
calculate the allocatable metadata chunk size, resulting 25.5GiB
available space.

But in reality we can only allocate one 1GiB RAID1 chunk, the remaining
49GiB on devid 2 will never be utilized to fulfill a RAID1 chunk.

This leads to various ENOSPC related transaction abort and flips the fs
read-only.

Now use per-profile available space in calc_available_free_space(), and
only when that failed we fall back to the old factor based estimation.

And for zoned devices or for the very low chance of temporary memory
allocation failure, we will still fallback to factor based estimation.
But I hope in reality it's very rare.

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

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