bcachefs: Fix want_new_bset() so we write until the end of the btree node

want_new_bset() returns the address of a new bset to initialize if we
wish to do so in a btree node - either because the previous one is too
big, or because it's been written.

The case for 'previous bset was written' was wrong: it's only supposed
to check for if we have space in the node for one more block, but
because it subtracted the header from the space available it would never
initialize a new bset if we were down to the last block in a node.

Fixing this results in fewer btree node splits/compactions, which fixes
a bug with flushing the journal to go read-only sometimes not
terminating or taking excessively long.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>