In the bch2_mount() error path, we were calling
deactivate_locked_super(), which calls ->kill_sb(), which in our case
was calling bch2_fs_free() without __bch2_fs_stop().

This changes bch2_mount() to just call bch2_fs_stop() directly.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>