bcachefs: key cache can now allocate from pending

btree_trans objects can hold the btree_trans_barrier srcu read lock for
an extended amount of time (they shouldn't, but it's difficult to
guarantee).

the srcu barrier blocks memory reclaim, so to avoid too many stranded
key cache items, this uses the new pending_rcu_items to allocate from
pending items - like we did before, but now without a global lock on the
key cache.

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