bcachefs: Ensure journal space is block size aligned

bcachefs: Ensure journal space is block size aligned

We don't require that bucket size is block size aligned (although it
should be!) - so we need to handle this in the journal code.

This fixes an assertion pop in jorunal_entry_close(), where the journal
entry overruns available space - after rounding it up to block size.

Fixes: https://github.com/koverstreet/bcachefs/issues/854
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>