btrfs: trivial conversion to return bool instead of int

Old code has a lot of int for bool return values, bool is recommended
and done in new code. Convert the trivial cases that do simple 0/false
and 1/true. Functions comment are updated if needed.

Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>