btrfs: make btrfs_block_rsv_full() check more boolean when starting transaction

When starting a transaction we are comparing the result of a call to
btrfs_block_rsv_full() with 0, but the function returns a boolean. While
in practice it is not incorrect, as 0 is equivalent to false, it makes it
a bit odd and less readable. So update the check to not compare against 0
and instead use the logical not (!) operator.

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