Full mark and sweep gc doesn't (yet?) work with the new btree key cache
code, but it also blocks updates to interior btree nodes for the
duration and isn't really necessary in practice; we aren't currently
attempting to repair errors in allocation info at runtime.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>