btrfs: rename the functions to count, test and get bit ranges in io trees

btrfs: rename the functions to count, test and get bit ranges in io trees

These functions are exported so they should have a 'btrfs_' prefix by
convention, to make it clear they are btrfs specific and to avoid
collisions with functions from elsewhere in the kernel.

So add a 'btrfs_' prefix to their names to make it clear they are from
btrfs.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>