bcachefs: Call bch2_fs_start before getting vfs superblock

This reverts

1fdbe0b1 bcachefs: Make sure c->vfs_sb is set before starting fs

switched up bch2_fs_get_tree() so that we got a superblock before
calling bch2_fs_start, so that c->vfs_sb would always be initialized
while the filesystem was active.

This turned out not to be necessary, because blk_holder_ops were
implemented using our own locking, not vfs locking.

And this had the side effect of creating a super_block and doing our
full recovery (including potentially fsck) before setting SB_BORN, which
causes things like sync calls to hang until our recovery is finished.

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