bcachefs: Make sure to drop/retake btree locks before reclaim

We really don't want to be invoking memory reclaim with btree locks
held: even aside from (solvable, but tricky) recursion issues, it can
cause painful to diagnose performance edge cases.

This fixes a recently reported issue in btree_key_can_insert_cached().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-by: Mateusz Guzik <mjguzik@gmail.com>
Fixes: https://lore.kernel.org/linux-bcachefs/CAGudoHEsb_hGRMeWeXh+UF6po0qQuuq_NKSEo+s1sEb6bDLjpA@mail.gmail.com/T/