sbin/hammer: Fix writeback against readonly fd

sbin/hammer: Fix writeback against readonly fd

writehammerbuf() needs to return 0 before it attempts to write
if the fd is readonly. Without this check flush_all_volumes()
may fail before all modified volume headers are written back
to the backing storage.

The only reason this hasn't become an issue so far is because
no existing hammer command creates readonly volume_info while
there are rw volume_info that need to be written back, which
could happen depending on the purpose of commands.