bcachefs: Fix for journal getting stuck

bcachefs: Fix for journal getting stuck

The journal can get stuck if we need to get a journal reservation for
something we have a pre-reservation for, but aren't able to reclaim
space, or if the pin fifo is full - it's impractical to resize the pin
fifo at runtime.

Previously, we reserved 8 entries in the pin fifo for pre-reservations,
but that seems small - we're seeing the journal occasionally get stuck.
Let's reserve a quarter of it.

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