btrfs: change BUG_ON to assertion when verifying root in btrfs_alloc_reserved_file_extent()

The file extents are normally reserved in subvolume roots but could be
also in the data reloc tree. Change the BUG_ON to assertions as this
verifies the usage assumptions.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>