bcachefs: Start snapshots before bch2_gc()

bcachefs: Start snapshots before bch2_gc()

bch2_gc may require snapshots to be started - the repair path when
checking the reflink btree may do updates to the extents btree.

This moves bch2_fs_initialize_subvolumes() and bch2_fs_snapshots_start()
to before bch2_gc() - since we haven't gone RW yet, the updates in
bch2_fs_initialize_subvolumes() are done via the journal replay keys
list, so it's fine to do this before bch2_gc().

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