Revert "btrfs: turn fs_roots_radix in btrfs_fs_info into an XArray"

Revert "btrfs: turn fs_roots_radix in btrfs_fs_info into an XArray"

This reverts commit 7b43ebbc2ebda688166cf5bcf38266573475ee19.

Revert the xarray conversion, there's a problem with potential
sleep-inside-spinlock [1] when calling xa_insert that triggers GFP_NOFS
allocation. The radix tree used the preloading mechanism to avoid
sleeping but this is not available in xarray.

Conversion from spin lock to mutex is possible but at time of rc6 is
riskier than a clean revert.

[1] https://lore.kernel.org/linux-btrfs/cover.1657097693.git.fdmanana@suse.com/

Reported-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>