bcachefs: Fix bch2_fs_get_tree() error path

When a filesystem is mounted read-only, subsequent attempts to mount it
as read-write fail with EBUSY. Previously, the error path in
bch2_fs_get_tree() would unconditionally call __bch2_fs_stop(),
improperly freeing resources for a filesystem that was still actively
mounted. This change modifies the error path to only call
__bch2_fs_stop() if the superblock has no valid root dentry, ensuring
resources are not cleaned up prematurely when the filesystem is in use.

Signed-off-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>