sbin/hammer: Cleanup blocks with a single statement

sbin/hammer: Cleanup blocks with a single statement

This commit basically does the same as what Linux kernel's coding
style mentions for braces, which is basically the same with BSDs.
https://github.com/torvalds/linux/blob/master/Documentation/process/coding-style.rst#3-placing-braces-and-spaces

No diff in sbin/hammer/hammer binary when assert(3) is disabled.