btrfs: make free_space_test_bit() return a boolean instead

btrfs: make free_space_test_bit() return a boolean instead

The function returns the result of another function that returns a boolean
(extent_buffer_test_bit()), and all the callers need is a boolean an not
an integer. So change its return type from int to bool, and modify the
callers to store results in booleans instead of integers, which also makes
them simpler.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>