bcachefs: Don't use key cache during fsck

bcachefs: Don't use key cache during fsck

The btree key cache mainly helps with lock contention, at the cost of
additional memory overhead. During some fsck passes the memory overhead
really matters, but fsck is single threaded so lock contention is an
issue - so skipping the key cache during fsck will help with
performance.

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