bcachefs: Fix write buffer flushing from open journal entry

bcachefs: Fix write buffer flushing from open journal entry

When flushing the btree write buffer, we pull write buffer keys directly
from the journal instead of letting the journal write path copy them to
the write buffer.

When flushing from the currently open journal buffer, we have to block
new reservations and wait for outstanding reservations to complete.

Recheck the reservation state after blocking new reservations:
previously, we were checking the reservation count from before calling
__journal_block().

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