bcachefs: Fix additional misalignment in journal space calculations

Additional fix on top of

f54b2a80 bcachefs: Fix misaligned bucket check in journal space calculations

Make sure that when we calculate space for the next entry it's not
misaligned: we need to round_down() to filesystem block size in multiple
places (next entry size calculation as well as total space available).

Reported-by: Ondřej Kraus <neverberlerfellerer@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>