bcachefs: Journal seq now incremented at entry open, not close

bcachefs: Journal seq now incremented at entry open, not close

This patch changes journal_entry_open() to initialize the new journal
entry, not __journal_entry_close().

This also means that journal_cur_seq() refers to the sequence number of
the last journal entry when we don't have an open journal entry, not the
next one.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>