bcachefs: Fix a slab-out-of-bounds

bcachefs: Fix a slab-out-of-bounds

In __bch2_alloc_to_v4_mut(), we overrun the buffer we allocate if the
alloc key had backpointers stored in it (which we no longer support).

Fix this with a max() call.

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